Support building other mines than biotestmine

The intention of the `--im-repo` flag is to build mines other than the default biotestmine. While the intermine_builder build.sh script is supposed to be universal, I doubt we'd be able to build a different mine right now without some fiddling.

We can start with [testmine](https://github.com/intermine/intermine/tree/dev/testmine), which holds fake employee data that is used for testing. Something tricky about this mine is it's not its own repo like biotestmine, it's instead part of the `intermine` software repo. This means we can't use `--im-repo` and need to manually clone it before calling intermine_boot with a path (see #29).