Running with no arguments should run all tests

[qrush]

Basically m with no args should just exec 'rake test'. Right now it's an error:

[master][~/Dev/rubygems.org] be m                                 
/Users/qrush/Dev/m/lib/m/runner.rb:4:in `initialize': undefined method `split' for nil:NilClass (NoMethodError)
    from /Users/qrush/Dev/m/lib/m.rb:13:in `new'
    from /Users/qrush/Dev/m/lib/m.rb:13:in `run'
    from /Users/qrush/Dev/m/bin/m:4:in `<top (required)>'
    from /Users/qrush/.rbenv/versions/1.9.3-p0/lib/ruby/gems/1.9.1/bin/m:19:in `load'
    from /Users/qrush/.rbenv/versions/1.9.3-p0/lib/ruby/gems/1.9.1/bin/m:19:in `<main>'