Generate prerelease compatible RPM version numbers and release numbers - 662

I think this should produce numbers that follow the guidlines here:
 * http://fedoraproject.org/wiki/Packaging:NamingGuidelines#Pre-Release_packages

There seem to be one or two warning when I build this locally with mock and test with rpmlint so it'll be interesting to see what buildbot says.

```
[~/.../HybridLogic/flocker-0.1.3pre1]$ rpmlint /var/lib/mock/fedora-20-x86_64/result/python-flocker-0.3.0-0.1.dev.fc20.src.rpm 
python-flocker.src: W: spelling-error %description -l en_US multi -> mulch, mufti
python-flocker.src: W: spelling-error %description -l en_US frontend -> fronted, front end, front-end
python-flocker.src:121: W: macro-in-%changelog %{version}
python-flocker.src:121: W: macro-in-%changelog %{release}
python-flocker.src: W: invalid-url Source0: Flocker-0.3.0dev1.tar.gz
1 packages and 0 specfiles checked; 0 errors, 5 warnings.
```

I tested this by adding various local tags to a local git clone of a clone to avoid any chance of pushing testing tags to GitHub.

Fixes #662 
Since this file is only of use with a source distribution, there is no reason for this to be something we install. My suggetion is to make `admin` a package and put it there. (Note that you'll need to make sure to add it to the sdist, by adding stuff to `MANIFEST.in`.

Also, since this is imported from `setup.py`, we need to ensure that it only imports things from the stdlib (and possibly setuptools). Please leave a comment to that effect.