Refactor Client to Faraday and modulize API.
Remove now-redundant code.
Reorganize usings, remove unnecessary lines.
Explicit module statement.
Convert tests to spec style.

Since Discourse uses spec style tests for Ruby code, I thought it
would be best to follow the same pattern for the API gem.
Change initializer to former API initializer.
Reorganize Client spec slightly.
Add connection options and user agent for encapsulation.
Convert from MiniTest to RSpec.

WebMock plays much nicer with RSpec that MiniTest::Spec, so this
seems prudent.
Add tests for Client.
Add real tests for categories and topics API methods.
Add Travis badge to README.
Make api_key and api_username accessible and add tests.
Add simplecov for code coverage.

Formatting with MultiFormatter, while unnecessary at the moment, will
allow us to add Coveralls with one line, should we choose.
Cover Client#post with a test.
Filter specs from simplecov.
Add test for API::Users.user and stub tests for rest of module.
Add support for topics by user.
Add support for search API.
Add Topics#new_topics.