Leap: Leap Motion Hand Tracking Cameras

This adds support for Leap Motion hand tracking cameras.

## Dependencies

Leap Motion SDK: The SDK is available for Windows and GNU/Linux on Leap Motion's Developer website. To build OpenSurgSim with Leap Motion support, create a LEAPSDK_DIR environment variable that points to the location of the LeapSDK directory. This directory should include the 'include' and 'lib' directories. Also, in CMake enable BUILD_DEVICE_LEAP.