Better relative path support

If a relative path is set, the plugin will check that this relative path
exists first before running any of it's functions. If the pat does not
exist, then it will gracefully default back to the value of
`$WORKON_HOME` first, then `~/.virtualenvs` last. This is is done based
on what vim's CWD is.

note: I think I squashed wrong because it wants to commit 01a52ab as well for some reason. But I haven't made any changes to that commit FYI so you can ignore it I guess.