ENH: Implement faster multivariate integration in scipy.integrate.

The numerical integration routines in SciPy use a well-tested, fast, Fortran
library. The computational cost of integration with these routines can be
greatly reduced if the function to be integrated is compiled and passed to
the library directly, avoiding callbacks to the Python interpreter.
TST: Preliminary implementation of unit tests for faster integration.
TST: Cross-platform build and install of unit tests.
DOC: Updated THANKS.txt and scipy 0.15.0 release notes.
DOC: Add Ctypes integration tutorial.
MAINT: fix formatting of ctypes section in integrate tutorial.
Merge branch 'pr/3262' into master.

Adds support for direct callbacks to C functions via ctypes
for multivariate integration.

Reviewed at https://github.com/scipy/scipy/pull/3262