Support for phpunit code coverage reporting.

PHPUnit can collect test coverage data, however, as many tests invokes a new drush process, it's not much use in Drush per default.

This PR adds three things:
 1. A switch to drush to collect coverage data to a file.
 2. Makes Drush_CommandTestCase use that switch to collect the coverage data from the run drush commands, and adding it to phpunits.
 3. A bit of patching around shell aliases.

Coverage can be generated by adding the --coverage-html=/path argument to unish.sh.