Add ruby code coverage with simplecov

coverage enables via environment COVERAGE.

    $ COVERAGE=YES rake test
        or
    $ COVERAGE=YES ruby install.rb test

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2633 f4f36d40-9719-0410-9868-dde021a23eda