WORKING addition of context scrolling

This change makes it possible to use scrollToBySpeed for elements with scrollable content (e.g., overflow: auto), instead of only on the whole window. I've tested three different use cases:

Full window (default) – http://codepen.io/cdCorey/full/Iexzc/
Fixed div with overflow auto – http://codepen.io/cdCorey/full/vgspz/
Static div with overflow auto – http://codepen.io/cdCorey/full/KsLxJ/

All seem to be working. They scroll either the window (in the first case) or the specified element (in the second & third cases) to the correct position, at the correct speed, regardless of both the window's and the element's scroll position at the start, going both up and down.

This feature is controlled with the new optional parameter "context". It defaults to the string "__window", which triggers the classic behavior (window scrolling). It accepts either a selector string or a jQuery object (e.g., either ".foo" or $(".foo")), in which case it will scroll within that context instead of the whole window.

Please feel free to do your own testing to confirm I haven't missed anything, but I believe this change can now be safely incorporated into the official code.