Depend on system install of libfreenect rather than ROS repositories

[maxsvetlik]

This fork targets ROS melodic and addresses how dependent files are referenced.

In previous releases of freenect_stack (like those for Lunar and Kinetic) it has depended implicitly on the version of libfreenect released by ros-drivers.

libfreenect and libfreenect-dev are also distributed in the Linux ecosystems. However, the way freenect_stack looks for header files prevents installations of libfreenect by apt and other installers, from being found. These installations place header files in /usr/include/ rather than /usr/include/libfreenect/, which is where freenect_camera assumes them to live.

This fork changes the dependency on the ros-drivers/libfreenect to the more broadly distributed libfreenect, which has an existing rosdep key of libfreenect-dev, and changes code to look for installed headers in place that the apt libfreenect installs them.

Considering the ros-drivers version of libfreenect is no longer maintained, and behind the latest release of the apt libfreenect, moving away from the ros-drivers version seems reasonable to me.

[maxsvetlik]

Again noting that this should be targeted for melodic and onwards, as merging it directly into master without freezing the current code to a kinetic branch may break some builds on earlier distros.

[maxsvetlik]

With travis now passing, I'll merge this and do a release unless anyone has anything else to say in the next 20 hours...

[maxsvetlik]

Closes #48. Will be releasing after merge to master.