Add bill query for managing bill searches, including pages

This pull request does 3 things:

1) Fixes requires that were inadvertently moved around in a previous commit
2) Adds the remaining result fields to bill.rb and adds a by_bill_id method
3) Creates BillQuery which allows full text searching of bills, high level filtering by fields, and includes support for pagination. An example of BillQuery usage:

```ruby
bills = Sunlight::Congress::BillQuery.search("health care")
#=> #<Sunlight::Congress::BillQuery:0x007fd864210b58 @uri=#<URI::HTTP...
bills.count   # number of results returned
#=> 3230
bills.results   # results in an array of Bill objects
#=> [#<Sunlight::Congress::Bill:0x007fd86401fda8 @title=nil, @bill_id="hr3590-111"...
bills.next_page!   # get the next page - returns results and shifts page in bills instance
#=> [#<Sunlight::Congress::Bill:0x007fd8642f3a70 @title=nil, @bill_id="s1459-111"...
bills.current_page
#=> 2
```

Thoughts?