updated Http Client Interface and small csfixes
updated Repositories/PullRequests::all to allow 'state' param
updated docs for PullRequests
More helper methods in TestCase

Added mocks for:

    - Transport client
    - Http client

Added `fakeResponse` helper method to build a Response object from given data.
added getClassMock method in TestCase
implemented repositories endpoint (API 2.0)
updated dockblock in Api.php
added post, put, delete in getHttpClientMock
keep it DRY
implemented PullRequests::create (API 2.0)
implemented PullRequests::update (API 2.0)
implemented PullRequests::get (API 2.0)
implemented PullRequests::commits (API 2.0)
implemented PullRequests::approve (API 2.0)
implemented PullRequests::deleteApproval (API 2.0)
implemented PullRequests::diff (API 2.0)
implemented PullRequests::activity (API 2.0)
updated PullRequests::activity to support fetching a single PR activity
implemented PullRequests::accept (API 2.0)
implemented PullRequests::decline (API 2.0)