uses versioneer to calculate version info from git tags

Hard-coding version information in the repository and remembering to update it to keep it in sync with git tags is a pain. This pull request uses a package called "versioneer" to calculate the version information dynamically, based on the most recent git tag and the number of commits since. So for example, the version for this commit f8518e2 is `0.3.0.dev-98-gf8518e2` (98th commit since tag 0.3.0.dev). If I did a `git checkout v0.3.0.dev`, `hexrd -V` would report the version as simply `0.3.0.dev`.

Once we tag `v0.3.0`, the next commit to take place should be tagged `0.4.0.dev` or `0.3.1.dev`.

Posting conda packages during the development cycle that report useful version information has not been possible until now. This pull request will make it easier for users to upgrade hexrd conda package development snapshots. We'll create a "dev" channel at binstar.org/praxes/dev and post development snapshots there. Users who want the latest developments can add that channel and do a simple `conda update hexrd` to get the latest development snapshot, since it will now contain sequential version information. We'll reserve binstar.org/praxes for official releases, as soon as 0.3.0 is ready to ship.