Add a version command.

Fixes #335
The git tag/revision is saved when running "make install", and can be displayed using "dokku version".

The version shown is the output from `git describe --tags`.
This consists of the closest tag, the number of commits since, and the sha1 of the revision.

If `make install` is not run in a git repo, then a hardcoded version defined at the top of the `Makefile` is used, as well as the installation date and time.
This version should be left to `master` on the master branch and set before releasing in the release branches.