Fail slow
Remove ClientError, ServerError, and ParserError

Closes #440.
Give Twitter module class-like interface
Remove other authors' email addresses
Remove token stub
Remove global configuration
Rename oauth_token to access_token
Update documentation on configuration [ci skip]
Move Twitter::Client into REST namespace
Alias access_token to oauth_token
Remove Twitter::Error#backtrace
Add Twitter::Token#bearer? method
Factor common code into Twitter::Client class
Add specs for passing in a bearer token
Add note about errors [ci skip]
Fix warnings
Add specs for passing URIs to #friendship
Remove dead code
Prune unused fixtures
Expose user agent on Twitter::Client