General:
* Further refinement to Vulkan support. Note: the interface is still subject to change.

IVRSystem (v14):
* ComputeDistortion signature change. Previous version used nan as error signaling. New version returns explicit success.
* GetHiddenAreaMesh takes optional EHiddenAreaMeshType argument to request Standard, Inverse, or LineLoop mesh.

IVRCompositor (v18):
* New submit error, VRCompositorError_AlreadySubmitted (if you submit the same eye twice)
* GetFrameTimings signature change
* Exposed GetCurrentFadeColor, GetCurrentGridAlpha

IVROverlay (v13):
* Added SetOverlayIntersectionMask

IServerDriverHost:
* Added PollNextEvent

IClientDriverHost:
* Removed GetSettings
* Added GetGenericInterface

ClientTrackedDeviceProvider (v5):
* GetHiddenAreaMesh takes additional argument, EHiddenAreaMeshType

Pull Requests integrated:
#48 Format README in Markdown
#58 correct detection of GCC
#68 helloworldoverlay: Specify c++11 to fix gcc errors
#76 Remove duplicate, unneeded glBindBuffer ca...