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: make some test directories into packages

A couple of missing __init__.py files.
RF: add setup_helpers.py to MANIFEST.in
BF: add more robust travis installs

Try a few times to install travis wheels before giving up.
Merge pull request #529 from matthew-brett/little-releasey-fixes

MRG: a couple of small cleanup fixes
BF: work round ugly MSVC manifest bug

Link a shared library rather than an executable to avoid nasty Windows
MSVC 10+ bug (http://bugs.python.org/issue4431).
Merge pull request #530 from matthew-brett/check-linking-with-lib

Great! thanks!
Merge pull request #528 from matthew-brett/conditional-omp-fix

MRG: move conditional compiling to C
Add needed subset of numpy functions and remove npymath library from linking
BF: add npymath info but remove the library from linking
BF: NPY_HAVE_DECL_SIGNBIT is true but _signgit is undefined in vc runtime
BF: check definition of signbit directly
RF: remove check_npymath
BF: explicitly request manifest creation to msvc
BF: /MANIFEST flag no longer needed