(wip) preserve existing dependency revisions

This changes the default behavior of 'godep save' so as
to preserve any dependencies already in the list at the
revision they were previously saved at. New dependencies
will be added, and old dependencies will be removed, but
that's all. This makes the save command roughly
analogous to 'go get' with no flags.

PENDING:
We also introduce a new command, 'godep update', to
update one or more dependency versions. This is roughly
analogous to 'go get -u'. This is a separate command
(rather than 'godep save -u') because the package
argument is different: it specifies a dependency rather
than a top-level package.

fixes #65