Added new properties

Added the following three properties:
@property(assign, nonatomic) BOOL longPressToEraseEnabled;
@property(nonatomic,readonly) NSUInteger vertexCount;
@property(weak, nonatomic)id<PPSSignatureViewDelegate> signatureViewDelegate;

This allows to get notified once the signature has changed. Also the vertex count can be read so you can e.g. decide whether a signature is valid / contains enough vertices. Also the long press to erase function can be disabled.