Adds support for simple nesting of describe blocks

Hey there,
Here is a small improvement to the `describe` BDD style of munit which makes it possible to nest describe blocks to generate tinytest "group paths".

```CoffeeScript
describe 'Running tests from within nested describe blocks', ->

  sharedData = {}

  beforeEach -> @suite.shared = sharedData

  it 'runs the first-level tests correctly', ->

    expect(@suite.name).to.equal 'Running tests from within nested describe blocks'
    expect(@test.test_case.shortName).to.equal "runs the first-level tests correctly"
    expect(@test.test_case.name).to.equal "#{@suite.name} - runs the first-level tests correctly"
    expect(@suite.shared).to.equal sharedData

  describe 'when nesting', ->

    it 'adds dashes to generate tinytest group paths', ->

      expect(@suite.name).to.equal 'Running tests from within nested describe blocks'
      expect(@test.test_case.shortName).to.equal "adds dashes to generate tinytest group paths"
      expect(@test.test_case.name).to.equal "#{@suite.name} - when nesting - #{@test.test_case.shortName}"
      expect(@suite.shared).to.equal sharedData

    describe 'multi nesting', ->

      it 'can handle any depth of nesting', ->

        expect(@suite.name).to.equal 'Running tests from within nested describe blocks'
        expect(@test.test_case.shortName).to.equal "can handle any depth of nesting"
        expect(@test.test_case.name).to.equal "#{@suite.name} - when nesting - multi nesting - #{@test.test_case.shortName}"
        expect(@suite.shared).to.equal sharedData
```
![screenshot of nested describe blocks](https://cloud.githubusercontent.com/assets/172414/4519102/200e6270-4cae-11e4-99e9-b66f5d3a3427.png)

This is not exactly how nested describe blocks work in jasmine / mocha – its still just a single suite but extremely useful for simple cases where you want to "logically" group sets of tests by adding tinytest "dashes" to the test names.