(maint) Add rake task for parallelizing specs.

Adding a 'parallel:spec' task that will group specs and then run those groups in a parallel set of processes.

By default, parallel:spec will bunch the specs into groups of approximately 1000 and concurrently run each group up to the number of processors reported by facter.

The task takes two parameters: process_count and group_size.

Running with a process_count of 1 (parallel:spec[1]) will behave like the existing 'spec' task, except it will still group the tests and recycle the spec running process between groups.  This has the added benefit of limiting the spec running process' working set, at the cost of being slightly slower (~10% when not performing any parallelization).