WIP: 3.0.0

## 3.0.0

- Changed ``twython/twython.py`` to ``twython/api.py`` in attempt to make structure look a little neater
- Removed all camelCase function access (anything like ``getHomeTimeline`` is now ``get_home_timeline``)
- Removed ``shorten_url``. With the ``requests`` library, shortening a URL on your own is simple enough
- ``twitter_token``, ``twitter_secret`` and ``callback_url`` are no longer passed to ``Twython.__init__``
    - ``twitter_token`` and ``twitter_secret`` have been replaced with ``app_key`` and ``app_secret`` respectively
    - ``callback_url`` is now passed through ``Twython.get_authentication_tokens``
- Update ``test_twython.py`` docstrings per http://www.python.org/dev/peps/pep-0257/
- Removed ``get_list_memberships``, method is Twitter API 1.0 deprecated
- Developers can now pass an array as a parameter to Twitter API methods and they will be automatically joined by a comma and converted to a string
- ``endpoints.py`` now contains ``EndpointsMixin`` (rather than the previous ``api_table`` dict) for Twython, which enables Twython to use functions declared in the Mixin.
- Added OAuth 2 authentication (Application Only) for when you want to make read-only calls to Twitter without having to go through the whole user authentication ritual (see docs for usage)
- Added ``obtain_access_token`` to obtain an OAuth 2 Application Only read-only access token
- ``construct_api_url`` now accepts keyword arguments like other Twython methods (e.g. instead of passing ``{'q': 'twitter', 'result_type': 'recent'}``, pass ``q='twitter', result_type='recent'``)
- Pass ``client_args`` to the Twython ``__init__`` to manipulate request variables. ``client_args`` accepts a dictionary of keywords and values that accepted by ``requests`` (http://docs.python-requests.org/en/latest/api/#sessionapi) [ex. headers, proxies, verify(SSL verification)] and the "request" section directly below it.
- Added ``get_application_rate_limit_status`` API method for returning the current rate limits for the specified source
- Added ``invalidate_token`` API method which allows registed apps to revoke an access token presenting its client credentials
- ``get_lastfunction_header`` now accepts a ``default_return_value`` parameter. This means that if you pass a second value (ex. ``Twython.get_lastfunction_header('x-rate-limit-remaining', 0)``) and the value is not found, it returns your default value

### To-do
- [x] Add file encoding to all files ``# -*- coding: utf-8 -*-``
- [x] Add file description to all Twython api files
- [x] Remove deprecated variables fixes #185 
- [x] Remove support for camelCase functions fixes #199 
- [x] Substitute `api_table` for actual functions (`EndpointsMixin`) fixes #211 
- [x] Better way to pass `requests` config fixes #213