Add AMD support

[alejandroiglesias]

This adds AMD support reverting to variable in global scope if not on AMD environment.

[oyvindkinsey]

There are some code paths that require looking up easyXDM on the global object (SameOriginTransport comes to mind) - could you review the implications for this, if it's acceptable, and whether some docs should be added for this?

[alejandroiglesias]

Thanks. Can you please provide more information about transports using easyXDM on the global object? Do they use the namespace if .noConflict() was called? Maybe if we're in AMD environment, we can also set the namespace and use that reference inside of the components that need to access the easyXDM object. (Or move to an event-based architecture? But i doubt to have the time to contribute for that.)