CMake and texture object support

[cpviolator]

This PR adds CMake functionality to the library to allow easier compilation up the dependency tree. It also offers some initial texture object support, retains texture reference and no texture use support.

Texture object and reference distinction is deduced at compile time, and texture use is configurable via:

1. The CMake option DEDISP_TEXTURE which can be set using ccmake GIU
2. The command line cmake -DDEDISP_TEXTURE ON
3. Setting the envar export DEDISP_USE_TEXTURE=1 at compile time.

NOTE: Texture creation and destruction is done wholly in the src/kernels.cuh function dedisperse, which may not be the most efficient. Feedback on this to address performance is appreciated and any required changed will be implemented.

TODO:

• Add installation functionality to CMake (61883d7)
• Check performance regression