Updated WP_Codebird::_callApi() to support _buildMultipart()

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.
Tidied up the code a bit
Removed exceptions and error suppressions, replaced output buffering with file_get_contents()

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()
Incremented version number and removed debug code.
Merge pull request #7 from MikePearce/master

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