Fix various Problems and add some features

[AlexanderSchuetz97]

Good day to you,

this pr fixes #1

fix various problems:

1. remove Cargo.lock from git repository, it was already in gitignore but still checked in.
   => I didn't check if the file is also present on crates.io release, but this file prevented using the crate as a git or path dependency during development with the latest rust compiler. It should be removed.
2. fix all generated certificates were always ca certificates regardless of input.
   => as discussed in the issue
3. fix all generated certificates always had the wrong signature algorithm nid used. They used the NID of the encryption algorithm not the signature algorithm. Most applications such as apache2 or rust-tls refused to load such certificates because they couldn't make sense of the signature.
   => This was a pain to find. You had a test, but the test did test it with the wrong NID. You checked for X9_62_ID_ECPUBLICKEY which is the NID for the public key itself, but the NID should have been ECDSA_WITH_SHA256 which is the actual signature algorithm. I only found this after side by side comparing a dearmored dump of the certificate with openssl
   with one generated by openssl itself. I have no clue what application even accepted the certificates that were generated by 0.1.0 but neither rust-tls nor apache2 would accept certificates with a bogus signature algorithm.
   Perhaps applications that do not check at all if the certificate signature is valid?
   Since I do not know a web server that accepts such certificates I couldn't check with a browser,
   but I doubt a browser would have accepted it.

added some convenience features:

1. add easy way to generate a self signed certificate with a custom validity duration that is not 1 year from now on.
   => Some of my systems have time desync with each other (yes I know skill issue on my end) but this caused the certificate to only become valid in 5 minutes because the client was 5 minutes behind in time, which caused ssl errors on the clients end. Providing a easy method that allows me to specify the validity allows me to just pass "yesterday" as start date.
2. add easy way to output the private key from the KeyPair to a pcks8 pem file.
   => Getting the private key either as DER or PEM from the KeyPair struct was a pain without this added function. You needed to add several dependencies to your crate with matching versions as well as do rather complex match statement with a lot of unreachable arms (depending on your choice of key algo). Adding this to certkit directly allows for very easy export of the private key.

Note:
I was unable to run the "botan" tests because botan(build-sys) did not compile on my computer.
So the "botan" tests may be failing. I have no idea what "botan" is.

I would appreciate it if this could be merged and if you could push a release after this.
I took the liberty of increasing the version for you already.

Sincerely
Alexander Schütz

[nacardin]

Thanks! I'll try to review soon

[nacardin]

Sorry, I was on vacation last week. I just merged and released on crates.io. Thanks for the fixes!

[AlexanderSchuetz97]

@nacardin
No worries, you still respond faster than some other maintainers.
Vacation is important too 😃

Thank you!