Use Versioneer for version management - fixes 73

Fixes #73.
In Flocker we also added a pinned `[dev]` dependency on versioneer, but I'm not sure why, since the module gets bundled. Maybe it's something you should do here too: 
 * https://github.com/ClusterHQ/flocker/commit/9a2d8ffdaff731baf6f39b5399005fb38852be16#diff-5