ENH: release the GIL for arr_insert inner loop.

Releases it only conditionally, as object arrays require refcounting to
be performed within the inner loop, making GIL release impractical.
REF: simplify multi-loop breaking with a goto.
ENH: Generate custom MSCV runtime library for mingw32.
ENH: Use customized MSVC runtime library if present.
ENH: Some commments. Skip customization for MSVC < 8.0.
ENH: Clean up symbol definition file.
ENH: In debug mode, do not use customized MSVC runtime library.
BUG: Also build MSVC debug library if MINGW_USE_CUSTOM_MSVCR is defined (best effort).
BUG: DLL finder should also look in WinSxS directory.
BUG: Add another preprocessor statement for 64-bits fallback mechanism.
ENH: Incorporated review advice and changed macro hack to MinGW hint.

When doing the typedef change, it conflicted with existing typedefs.
I realized that MinGW did not know which MSVC runtime it would link with,
but it had conditional definitions for MSVC 8 and up, which can be activated
by defining __MSVCRT_VERSION__. So I added it to the compiler macros, which
made the fallback hack and extra typedef unnecessary.
BUG: Fixes for Python3 and some further enhancements.
BUG: Fix version comparison bug on Py25.
DOC: describe required version of GCC in MinGW on Windows in release doc.
BUG: core: fix compilation for UCS-narrow
DOC: mention that NpzFile instances must be closed.
ENH: add context manager for NpzFile class.
BLD: fix up detection of Intel CPU on OS X in system_info.py.
ENH: Add 'pgfortran' as a valid compiler in the Portland Group
suite of fortran compilers. Adjust the version string command.
ENH: Explicitly coded argmin for timedelta