fixing composer dependency of codebird-php as wp-codebird only supports ...

We can now pass in either a image path or an image URL when sending
media assets to Twitter. Media filesystem access on WP.com is severely
restricted so this helps us work around that limitation.

fixing codebird-php dependency maintainer

Thanks @kevinlangleyjr!

Adding Composer Support

Fix for issue #6. Added _buildMultipart() for successful posting of images to twitter

As we hide Exceptions from the end user, I've changed it to _doing_it_wrong(), so even if there is a problem, it'll be picked up in development.
The errors were supporessed because if image data was passed as a string, the file_exists() and is_readable() don't pattern match, they look for a file. So complain they were passed a string and not a file path. The best way around this is to look for a supported file extension first.
Output buffering seems overkill when you're only looking for the content of one file. So changed to file_get_contents()

This fixes issue #6. Added support for _buildMultipart() which posts images to twitter correctly. Until we update Codebird on wpcom, this will allow posting of images. Essentially, it just looks to see if it's a multipart posting and, if so, builds the multipart post body (with the content-disposition tags and boundaries). It also set's the headers correctly.

Previously, some POST requests were not being signed correctly. This
refactors how _callApi() works to properly sign GET and POST requests.

Most Exceptions are thrown from API timeouts - which are more difficult
to find in testing.

To prevent fatals in production, this transparently handles Exceptions
in __call(), returning an empty array instead.

Room for improvement - store the Exception information as a WP_Error in
a protected var.

- Determine what the expected response type is. For example, status
updates don't expect an array response - an appropriate response should
be returned for the request.

Now supports app only authentication via Bearer