Add a version command.

The git tag/revision is saved when running "make install", and can be
displayed using "dokku version".
Use a hardcoded version if git revision is not available.
Add the date and time of installation if version is hardcoded.
Merge pull request #334 from plietar/version

Add a version command.
Merge branch ‘master’ into v0.2 and update DOKKU_VERSION