Use relative files in helm-eproject

I've been using eproject and helm for a long time now and lately I've been annoyed with the fact that helm-eproject sources does not show file relative paths for the current project in the helm buffer. 

This has been fixed now.

Possible improvements:
* make it configurable whether to display a file's relative path or its basename? -> helm let's you configure the behaviour for most of its completion buffers, e.g. helm-source-recentf, see helm variable `helm-ff-transformer-show-only-basename`. This would require an additional check in the `action` part of helm-eproject's source definition.
* add tests

**Note**: Change has only be tested on an OSx machine.