Move to Faraday and ditch rest client & mechanize.

TODO: Make some of the tests more robust.  They currently all pass though
Pinterest is now delivering search results in batches of 25 instead of 50.

This broke the tests but the implementation is still good.  Tests/cassettes have been updated.  If this isn't a one time thing and Pinterest dials the pagination count up/down in the future…  I'll rewrite the 3 impacted tests using a different approach.
Refactor the Client class' ability to make connections to Pinterest.

Broke things out into individual (private) methods to isolate some of the "complexity" and improve the test suite.