ENH: add N-dimensional integration function.

New n-dimensional integration routine 'nquad' included in scipy.integrate.
MAINT: some code formatting changes in integrate.nquad.
MAINT: less in-place modification
MAINT: less complicated variable args
MAINT: merge two functions
BUG: recursion termination condition
MAINT: fix documentation formatting and PEP8 for integrate.nquad code.
DOC: fix a few issues in the integrate examples.
BUG: fix nquad issue for depth=1 integration.  Add tests against dbl/tplquad.
DOC: improve clarity in integrate.nquad docstring.
DOC: add integrate.nquad to 0.13.0 release notes.
Merge branch 'pr/2510' into master.

Reviewed as https://github.com/scipy/scipy/pull/2510