#18 rollback functionality

[Rob Helmer]

it would be really cool to have a VCS command called
"rollback" that would allow the user to rollback X
number of revisions on a file or fileset, on that
file's current branch.

This is different than just updating to a revision, in
that it would be recorded as a local modification to
the current revision ( in CVS this can be done by doing
"cvs update -pr rev filename > filename" )