Explicitly state Site Streaming API has limited access

Site Streaming API access is limited. This is not immediately
clear in the documentation/comments of the gem, although it
is clear in the twitter API. Adding a @note similar to the firehose
streaming API will help clear up any confusion that the
developer is misusing the API through the gem. Also added
restriction information to README for clarification.
Add description to Twitter::Error
Merge pull request #509 from domitry/spike

Add description to Twitter::Error
Merge pull request #507 from eileencodes/explicitly-state-site-streams-have-restricted-access

Explicitly state Site Streaming API has limited access
Make Twitter::Error#wrapped_exception interface consistent with Ruby 2.1
Rename threaded_ methods to parallel_

Threads are an implementation detail. See discussion with @charliesome:
https://github.com/sferik/twitter/pull/503
Add conversion methods to Twitter::NullObject
Prefer module function to class << self
Update boilerplate at the top of each example [ci skip]
Prepare for version 5.5.0 release
Merge branch 'master' of https://github.com/sferik/twitter into helloworld
Search.md, better version of HelloWorld.md