Move polygon during edit mode.

Adds extra handle to center of polygon while in edit mode. Works similar to the move handle shown for editing a circle or rectangle.

Closes: https://github.com/Leaflet/Leaflet.draw/issues/243

I'm not sure where to update the tests. Is there a contrib file anywhere so I can do so?
```
$ jake
Checking for JS errors...
	Check passed.

Checking for specs JS errors...
	Check passed.

Running tests...


PhantomJS 1.9.7 (Mac OS X) ERROR
	TypeError: 'undefined' is not an object (evaluating 'L.Edit.SimpleShape.extend')
	at /Users/willfarrell/Sites/MMC/WebApplication/src/scripts/lib/Leaflet.draw-0.2.4-dev/src/edit/handler/Edit.Poly.js:7
PhantomJS 1.9.7 (Mac OS X): Executed 0 of 0 ERROR (0.144 secs / 0 secs)

Concatenating and compressing 24 files...
	Uncompressed: 73.32 KB (+3794 bytes)
	Saved to dist/leaflet.draw-src.js
WARN: Dropping unused function argument undefined [?:9,29]
	Compressed: 45.38 KB (+2027 bytes)
	Saved to dist/leaflet.draw.js
	Gzipped: 9.98 KB (+466 bytes)
```