Corrected and Expanded Documentation for tweet, user and REST::lists
Rescue Twitter::Error::NotFound for safe #favorite and #retweet

Closes https://github.com/sferik/twitter/pull/584.
Merge pull request #586 from andreykul/master

Corrected and Expanded Documentation
Cleanup after #586
Make Twitter::User#profile_background_image_uri methods return a URI
Enforce code coverage minimums
Return a Twitter::NullObject for empty attributes
Return a Twitter::NullObject for empty attributes
Return a Twitter::NullObject for empty attributes
Ensure predicate methods return false for Twitter::NullObject
Replace Faraday with the HTTP gem
JSON keys are parsed into strings, not symbols