Describe only

It's holiday season and tis the time to give, thus this pull request.

This is a mod of @vojtajina's ddescribe/iit pull request (https://github.com/pivotal/jasmine/pull/181)

The difference is I've renamed `ddescribe` to `describe.only` and `iit` to `it.only`, following Mocha's syntax. Tests are included. My hope is that this has a higher chance of getting merged in given that

1. it has prior art: see Mocha.
2. it does not introduce any additional globals.

Cheers!