Automatically download and extract libstemmer_c.tgz if libstemmer_c directory doesn't exist

My attempt at making it a little easier to build for people who've checked out the source code from Subversion or GitHub and don't have a `libstemmer_c` directory.

This will display a warning advising the user to add `bootstrap` to their `setup.py` invocation. The `bootstrap` "command" will download and extract the tarball for them.

This could've also been done with something like Paver or buildout, but I didn't think this was worth the complexity of adding more tools.