Added Twitter::SearchResults#next_results

Returns a hash of query parameters for accessing the next page of search results
Can be merged into the previous search options for easy access
Implement oauth2/invalidate_token
Merge pull request #372 from terenceponce/invalidate_token

Implement oauth2/invalidate_token
Adding Bearer Token support

- Added :bearer_token to the list of configuration parameters
- The default value of the bearer_token is read from the environment
  variable
- Requests send the "Bearer AAA..." type authentication header if a
  bearer token exists
- Added tests to verify that bearer token implementation works as
  intended
Adding oath2/token method

- I had to replicate the functionality of the request method to
  accomodate for the abnormal way we have to make the request
- Configurable can supply the base64 encoded bearer token credential
Moving methods around and added documentation.
Adding tests for the token method
Unifying the request method of Twitter::Client

- The request method is using a helper method request_setup that returns
  a Proc which sets up the correct headers for the current request.
- Each type of different authentication header is being returned by a
  separate private method and they all have tests.
Adding bearer_token as an alias to token
Removing left over code.
Unindented 'end' statement
require statement in the wrong file.
Merge pull request #387 from paracycle/bearer_token

Adding Bearer Token support
Merge pull request #365 from KentonWhite/next_results

Add Twitter::SearchResults methods for handling next results
added support for Symbol entities in Tweets

https://dev.twitter.com/blog/symbols-entities-tweets
rename token method to app_token, remove alias which collides with attribute in configurable
expose credentials hash to public, boolean methods for presense of user/bearer tokens
allow for :app_auth option, fallback to app_auth when no oauth_token present, lazy load bearer_token if needed
stub token request before each spec
clean up logic