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
Un-deprecate Twitter::Base#to_hash
Add Twitter::Tweet#possibly_sensitive?

Closes https://github.com/sferik/twitter/issues/587.
Use %q only for strings that contain both single quotes and double quotes
adding a note about a performance issue regarding DNS settings
Merge pull request #592 from kamerblauwlicht/performance_note_documentation

added a note about DNS problems causing multiple Twitter::Error::RequestTimeout errors
Lock rest-client to ~> 1.6.0 on Ruby 1.8.7
Modifed how geo is represented in Tweet
Merge remote-tracking branch 'upstream/master'