Add net/http timeout, io.Error, and random success/failure support fo…

[ljluestc]

This PR addresses issue #44 by adding:

• Timeout simulation with Response.Timeout to mimic net/http timeouts.
• I/O error simulation with Response.IOError for errors like io.EOF.
• Random success/failure with Response.Randomize (30% failure chance).

Tests in response_test.go verify the features. All files formatted with go fmt.

Note: Issue #44 was filed under xjdrew/gork but applied to h2non/gock based on context.