Dev versioning is broken.
currently:

```
0.12.0.dev-73de5de
```

[PEP386](http://www.python.org/dev/peps/pep-0386/) details 3 version format conventions extent in the python world and this format is invalid in all of them.

In practice it's caused no problems( I know of), probably because the tooling is lenient, but one real drawback is that lexi. sorting of the version string does not properly order the revisions, which some tools rely on. Something which

```
0.12.0pre234(-73de5de)
```

would provide, where 234 is the commit count since the last release. We could add a pd.version.commit field to compensate for omitting the commit in the version string.

As always it'll break something for sombody, but being a dev-only thing we're probably entitled.
