Add capturing screenshots to SLDevice
Adds SLGeometry, to be used for converting CGRects, CGPoints and CGSizes to their JavaScript counterparts, and vice-versa. Updates existing code to use this.
Document the screenshot methods of SLDevice, and organize the SLDevice header a bit with pragmas.
Document SLGeometry (this won't actually work on Appledoc because it doesn't support C functions -- but it's helpful when reading the header file, and hopefully can be used with Clang's new documentation support)
Add SLGeometryUnitTests and SLGeometryTestViewController