Remove #connect, #connected?, #disconnect, #update_tips! from Remote.
Change Remote#ls to connect and disconnect the remote automatically.
Implement Remote#fetch and update Repository#clone_at.

This moves and re-uses most of the existing Repository#clone_at option
handling code for Remote#fetch. It also implements correct handling of
the :progress and :update_tips callbacks.
Add a test for the new Remote#fetch.
Add tests for the :update_tips callback of Repository#clone_at.
Allow specifying an alternative list of refspecs to use in Remote#fetch.
Fix a doc typo.
Add a convenience Repository#fetch method.
Add a fetch example.