Issues/1792 support for uploading videos

#1792 
Added supported for uploading videos with the following caveats:

- Because the REST API still doesn't support the upload of Videos, I made the app force the use of the XML-RPC api when uploading videos

- Every time when I enter the post view I check if video press is available on the blog. This check should move to a place where blog info is sync from the server.

- I think I found a bug on the WPXMLRPCEncoder. When using this on a HTTP with a body in streaming mode. The encoder was deleting the streaming cache file on dealloc before it could be used on the actual request. For now I just commented out the deletion because these files are stored in the cache folder and will be deleted by the system when space is needed.
We could start the spinner on the navbar immediately if there where assets selected. Then have the Media Service callback to this class to say how many export/convert operations are completed and remove them one by one from the videosToOptimize array.