Output test coverage metrics to the metric_fu data directory

[bf4]

per #226

Given, tests are run win SimpleCov running
And the formatters include SimpleCov::Formatter::MetricFu
When the current date is 2014-05-04
Then the coverage_file_path should be set to output a coverage report to MetricFu.data_dir](https://github.com/metricfu/metric_fu/blob/d38ca22195a60fd7b5a30810a87e0eaba2bd2f0b/lib/metric_fu/io.rb#L43) With the name 20140504_coverage.json Where the prefix, 20140504 is the [Metricfu.report_id`

In this ticket, it may be decided whether the below should be done. If so, they should be in individual commits.

1. to completely remove outputting an an 'rcov.txt' to 'coverage/rcov/rcov.txt'
2. All Rcov name spaces should be renamed to TestCoverage (while providing backwards compatibility with the old :rcov configuration options)
3. Add deprecation warnings for reading in rcov.txt