Add executable and reconfigure Rake Task to use it

[wied03]

Work more like RSpec does where the Rake task calls the executable

[wied03]

Sort of related to https://github.com/opal/opal-rspec/issues/41

[hmdne]

While we don't call the RSpec binary in Opal-RSpec 1.1.0, the code of the executable, the rake task and the runner class have been mostly unified. Also ,we actually run Opal in a separate process.