Make CGMutablePathRef publicly available

[To1ne]

It would be nice if you could access the CGMutablePathRef of the ACEDrawingPenTool from the outside (e.g. to store it in a database).

I suggest overriding the CGPath accessor defined by UIBezierPath to do this. This should make things easy, because UIBezierPath conforms to NSCoding, so everything should work out-of-the-box.

I'll see if I find some time to format a PR to accomplish this.

[acerbetti]

I you have time, i would recommend to use the NSCoding protocol. See my comment to #65