Proper (un)escaping in URLVariables

 Including `+` => ` ` and vice versa.
Implement AVM1 loadVariables

Bonus: rudimentary support for loadMovie, too. Fleshed out in follow-up patches.
Support for AVM1 getURL

Not tested, because that'd involve setting up a server to verify that a request is being made and all that. Might well work, though.
Incomplete support for AVM1 loadMovie

The AVM1 parts *should* be done, but there are issues in Loader preventing this from working:
- Loader assumes that all loaded content is part of the display list when trying to construct the first frame
- the Loader instance doesn't have an AS2Context, and no way of getting one, because it doesn't have access to the stage without being part of the display list
- the initialization code is a very tasty, large bowl of Spaghetti Bolognese
Merge pull request #1161 from tschneidereit/master

AVM1 loadVariables and getURL