Add multicontext example (closes #19)

Add multi context example, tested on Ubuntu 14.04 with Nvidia 337.

The current output is
```
> primusrun ./build/multicontext 

Context 1
OpenGL Version:  3.2
OpenGL Vendor:   NVIDIA Corporation
OpenGL Renderer: NVS 4200M/PCIe/SSE2

Try to call glDispatchCompute(...);
Failed

Context 2
OpenGL Version:  4.4
OpenGL Vendor:   NVIDIA Corporation
Try to call glDispatchCompute(...);
Failed
```

That may be because the library exposes all function pointers, disregarding the requested context (which is documented behavior on Linux using X).

Please test the example and post its output for Windows and OS X machines.
On Windows, there should be an invalid function call during the first context.
On OS X the second context should be a 2.1 context and the first context should be the maximum valid version for your OS X (e.g. 4.1 on current OS X), but the function pointer should not be resolved in either case.