Adding Bluetooth mesh

[h2zero]

Just wanted to bring attention to the new mesh branch.

I've been working on adding a mesh API. So far basic on/off server and level models have been implemented and working great with nRF Mesh.

This is very much a work in progress, many features still need to be implemented and many models to add.

There is a demo in the examples folder. If anyone would like to test it out and/or contribute please feel free to do so, I'd really appreciate it!