Add Quaternion class as quat

Change include order to follow common conventions
Add trailing underscore to all include guards
Add quat
- .h and .cpp with mostly empty implementations
- quattest.cpp in the test which currently just
  checks if quat() == quat() (identity quaternion)
- Add to build system
Change version number to 0.8.2
Implement quaternion methods

Fix quat identity (1, 0, 0, 0) -> (0, 0, 0, 1)
Implement multiplication
Implement angleAxis rotations
Implement rotation methods
Add quat tests
- Test vector equality with (v cross v) == 0
Add lerp to all vectors

Change version number to 0.8.3
Restore compiling pkgconfig .pc file