Consider replacing "commit" with "changeset" & reserve "commit" for the action

I realize you get to paint the bikeshed as you built it, but at least give this some consideration.

It appears you are currently using git terminology. Now obviously you have to use something, but as was recently pointed out [1](http://groups.google.com/group/git-python/browse_thread/thread/a40163e6801d75ef?hl=en#) by someone new to git (on the gitpython mailing list): "commit" is an action, yet the "commit" method refers to a previous commit object. Perhaps "changeset" (as used by Mercurial and others) would be a better term so that "commit" can be reserved for the action of creating a new commit if that feature is ever added to pyvcs.

Now, I realize pyvcs is described as "lightweight", so creating comits may be beyond its scope, but with multiple backends supported by default, there shouldn't be any reason to be married to the terminology of any one specific backend. Rather, the terminology used should be clear to anyone familiar to any one of the supported backends.