Rename draft doc

This pull request adds the DraftRename command that uses the UPDATE command to send a new document name to draftin.com. You can now also optionally name the document when calling :Draft (whether at creation time or when updating).  

In the process, various things were fixed, such as updates will now only be sent if the buffer is modified. I also refactored more to make it cleaner to have more than one command function, and one problematic escaping problem (reading about shellescaping in vim, I'm thinking that more such problems may crop up if the plugin gains additional users).