Corrected return type of methods.

Methods that use the parse_encoded_uri method return an Addressable::URI not a String.
Merge pull request #583 from andreykul/master

Corrected return type of methods.
Add iso_language_code attr_reader to Twitter::Metadata
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