Colorize TestUnit spec output

RubyMotion's spec output is a bit dreary -- it works, but it is dense and hard to scan.

This pull colorizes a few aspects of the TestUnit output to more closely resemble that of other Ruby test frameworks: 
* passing specs are denoted by green period
* failing specs are denoted by a red F
* test count line is red or green depending on the passing state of the specs
* the test duration has been truncated to two decimal points
* more whitespace has been added

I'd like to colorize the failing specs as well, but that requires more significant code changes.