Split DataReaders.jl into several packages

[femtotrader]

When a large number of data sources will be available, it could be worth to split DataReaders.jl into several packages.

DataReaderBase will be a base package which will be used by other packages
(abstract type but also convenient functions which can be shared accross several package will be stored in DataReaderBase)

DataReaders.jl will be a package which will install all available data sources

DataReaderGoogleFinance
DataReaderYahooFinance
DataReaderIEX
...

Each package will be stored as a repository belonging to https://github.com/JuliaDataReaders GitHub organization