Add repository search

I am not the happiest with how the endpoint behaves, but I have an idea
Add support for text-match header
I am far happier with this
Change test
Re-record cassettes and modify tests

- Use token auth
- Move the assertion that actually makes the request inside the context
Add github3.search_repositories
Update the doc-strings
Make *Iterators accept headers
Make use of the new headers available
Display query in SearchIterator repr
Add integration tests for public search API

- Allow IntegrationHelper.cassette_name to take a class name for the cassette generation
Improve the documentation per @esacteskab's review
Fix #181

Never monkey with _json. Not even once