Allow introspection of configuration values.

Add a 'keys' method to capistrano/configuration. This method can be used
to introspect on all currently set configuration keys to allow
configuration dumps, for instance.

Improved from #958 and initial idea from #957.

I have squashed everything in one commit, rebased with master and reworked the commit message and description block. Also added an entry to the changelog under master. I checked the tag comparison and it also looks good to me. Anything else?