Add support for new REST API endpoint for bulk lookup of Tweets by ID

https://twitter.com/TwitterDev/status/459383946803830784
Refactor to use Hash#fetch
Make the streaming API raise exceptions for status codes.
Merge pull request #558 from ericroberts/handle-response-codes

Make the streaming API raise exceptions for status codes.
Call GET users/show if screen name has already been fetched

Closes https://github.com/sferik/twitter/issues/561.
Test against latest Ruby 2.1 release
Use (immutable) user ID instead of (mutable) screen name
Ensure predicate methods return false for Twitter::NullObject
Replace Faraday with the HTTP gem
JSON keys are parsed into strings, not symbols