Development version number

Under our current use of version numbers, it is not immediately obvious whether someone is using the `0.2` release branch or is running a more recent, development version.  This PR updates the version of our development branch to `0.3-dev`.

I envision that one of the last commits before the actual `0.3` release will update this version number to `0.3`, and then one of the first commits added to the development branch should further update the version number to `0.4-dev`.

(On an unrelated note, I also added more more temporary files to `.gitignore`.)