BDD style semantics ("describe" and "it").

As per issue: https://github.com/spacejamio/meteor-munit/issues/6

With this PR we now have Mocha style testing semanics within Munit:

![image](https://cloud.githubusercontent.com/assets/185555/3462452/a07d56c6-022d-11e4-8629-86b658b47b25.png)