Switch k4a and k4abt CMake to Module Mode

[skalldri]

Fixes #54

Description of the changes:

• Add a CMake Find Module for k4a and k4abt, which provides a cross-platform way of locating the k4a libraries
• Revamp CMake installation logic to install all dependencies of k4a, k4arecord, and k4abt
• Bump CMake minimum version to 3.5 to match Ubuntu 16.04

Before submitting a Pull Request:

• I reviewed CONTRIBUTING.md
• I built my changes locally
• I tested my changes with a device

I tested changes on:

• Windows
• Linux

[skalldri]

@RoseFlunder : this change adds better support for locating the various k4a libraries and dependencies, which should help out with your other changes!

It also adds automatic detection of the body tracking SDK: if discovered, it defines K4A_BODY_TRACKING, which you should be able to detect with

#if defined(K4A_BODY_TRACKING)

.... body tracking code goes here

#endif

[skalldri]

Passes build tests on Linux and Windows