NF: add safe openmp header / Cython pxd

Do check for openmp with C preprocessor, and then use C variable checks
to avoid using openmp constructs at build time.
RF: write config.h rather than config.pxi file

Signal that code should use C-level conditionals rather than
Cython-level conditionals by writing .h file instead of .pxi.
BF: include_dirs for Cython pyx release compiles

Add the `src` include directory to allow all (bundlemin in fact) Cython
files to be compiled into the source release tarball.
BF: add more robust travis installs

Try a few times to install travis wheels before giving up.
Merge pull request #528 from matthew-brett/conditional-omp-fix

MRG: move conditional compiling to C