preserve existing dependency revisions

This changes the default behavior of 'godep save' to
preserve any dependencies already in the list with their
existing revision. 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.

fixes #65
new command: update

This introduces a new command, 'godep update', to update
one or more dependency versions, roughly analogous to
'go get -u'. It 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.