Support for Multiple Windows in AIR

[benbjohnson]

Melomel currently only works with the main window. Add support to use the active window's stage when AIR is used.

Requirements:

• Default find() root to the active window's stage.
• Add a Cucumber task to activate a window.

[benbjohnson]

Added active window support to find() methods in v0.6.3. Moving Cucumber task requirement to Melomel.rb project.