Tentative implementation of connecting to an existing instance
Change Vimrunner::Server to take a hash of options.

This makes passing an executable and name optional.
Add Vimrunner::Server#connect for existing servers.

This allows clients to block much like Vimrunner::Server#start but
without risk of spawning a new server.
Add Vimrunner.connect.
Update README.md to mention Vimrunner.connect.
Merge pull request #13 from AndrewRadev/connecting-to-existing-server

API for connecting a client to an existing server by name