Feature: Add a unloadSuites method

There should be a way to unload currently loaded suites. 

I am working on a taskrunner for jasmine which as a feature should be able to run multiple sets of specs with the same instance of jasmine.

Adding a unloadSuites would make this possible.

Targeted flow is:

1. Load setA of specs 
2. Call jasmine.execute()
3. jasmine.unloadSpecs()
4. load setB of specs
5 call jasmine.execute()