Migrate Project.billable to Project.is_billable

[toolness]

This fixes #37.

I used a blog post on Writing Unit Tests for Django Migrations to test the migration.

Note that this also renames billable to is_billable, partly because this is in keeping with the convention of naming booleans (I thought this was part of PEP 8 but I guess it's not). We can add another migration that actually renames it back to billable if we want, though.

[toolness]

This migration feels weirdly over-engineered for such a seemingly simple change, but I wanted some experience writing testable migrations so that future, more complex migrations are easier to deal with. Hope that's ok!

[brendansudol]

very cool - i had seen a django migration test until now!