Support for distributed builds

Hey Ryan, I hacked this up to support distributed builds. Since phantom_jasmine already allows you to group tests onto different threads, this adds the ability to specify which groups will be run on each machine like this:

Machine one:
```bash
JASMINE_PARALLEL_COUNT=6 JASMINE_SELECT_GROUPS='1,2' bundle exec rake jasmine:phantom
```
Machine two:
```bash
JASMINE_PARALLEL_COUNT=6 JASMINE_SELECT_GROUPS='3,4' bundle exec rake jasmine:phantom
```
Machine three:
```bash
JASMINE_PARALLEL_COUNT=6 JASMINE_SELECT_GROUPS='5,6' bundle exec rake jasmine:phantom
```

This is very similar to [parallel_test](https://github.com/grosser/parallel_tests/wiki/Distributed-Parallel-Tests-and-Travis-Support) for RSpec/Cucumber/Test::Unit.