Base class to handle common API requests

API requests involving filtering (search) and pagination are used
across Sunlight resources. This introduces a base class to handle
these common methods.

Dependent classes create an instance of ::Base, passing themselves to
::Base, so we can get the calling class. We use calling class to
determine the Sunlight method to hit, like /legislators.

The response is parsed here as well–eventually I'd like to make this
smarter and DRY up the response parsing throughout the codebase.
Move configuration settings into Base

Moved getting/setting of base URI and API key into base, and added class helper methods to get/set them. Updated all the classes and tests to use Sunlight::Congress::Base to get base_uri and api_key
Legislator init update, support for field searches

Support legislator searches with multiple field filters (those are listed here: http://sunlightlabs.github.io/congress/legislators.html#fields).

Searches as passed in hash-style:

Sunlight::Congress::Legislator.search(state: "CA", chamber: "senate")

…which returns an array of Legislator objects. Instead of explicitly listing Legislator attributes in the initialize method, set these up as instance variables that passed in as a hash. Will add all the Legislator attributes in next commit.
Add all available Legislator attributes

Also added newline to end of base.rb, had forgotten to do that originally.