adds travis files & gearshifft version. float16 now as opt-in. fftw no-thread api.

[tdd11235813]

• adds travis files for continuous integration support (tested on trusty)
  • clfft, fftw and cufft are built with clang and gcc-5, while clfft and fftw are executed as travis CI does not support GPUs
  • fftw has a no-thread api as travis' clang does not work with OpenMP out of the box, so fftw has to work with libfftw3.so & libfftwf3.so
  • to get travis to work, I can send a request to mpicbg-scicomp to access mpicbg-scicomp's public data via travis CI or you set up an account on travis CI for mpicbg-scicomp/gearshifft
  • after travis CI is setup another commit must happen to start the build process (so I haven't added a travis CI logo yet)
• float16 is now optional and can be enabled via cmake parameter GEARSHIFFT_FLOAT16_SUPPORT
• adds a gearshifft version ("0.3.0")
• boost 1.59.0 is minimum requirement now (using 2nd parameter of auto test case and asio)
• adds a cmake message that downloading of half-code might fail when cmake's libcurl cannot handle https urls

[psteinb]

It'll take me some time to review this PR. If you wanna skip that time, could you please create issues for the TODOs and assign them to me. thanks.

[tdd11235813]

I know you are completely occupied this week, but if you will have some time next week, I would wait and looking forward to your review because after this PR and some minor fixes I want to bundle a new release. After that release a code cleanup+separation and probably your MKL integration could be part of the development.

The travis integration probably should be done by yourself as a mpicbg-scicomp maintainer directly. We can talk about it next week, I'll set up an issue for that.

Edit: See issue #116

[tdd11235813]

you now can merge if you want or test this PR by benchmarking the GTX 1080, which is required in the gearshifft_results repository.