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
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
Destructive implementation of symbolize_keys! is about twice as fast