Fix #88

This pull request adds two new flags: -pass and -silent.

-pass causes godeps to simply pass all `godep go [args]` commands directly through, rather than printing an error message.
-silent causes godep specific output to be silenced for `godep go [args]`

This change makes it easier to use godeps in various tools or "run on save" commands. godeps can now be called and the caller doesn't have to worry about whether or not godeps is in use for that package.