Core: Avoid short read failures on broken descriptors

* See http://marc.info/?t=135132844600001&r=1&w=2
Autotools: Carry over 2012.10.23 libusb changes

* Use LIBS instead of PC_LIBS_PRIVATE
* Move THREAD_CFLAGS & VISIBILITY_CFLAGS into AM_CFLAGS
* Use AC_SEARCH_LIBS(clock_gettime) for pthreads on Linux
Tests: Add libusbx stress test

See https://github.com/tobygray/libusbx/tree/testing as well
as http://libusbx.1081486.n5.nabble.com/Libusbx-devel-Crashes-tt433.html#a438
Core: Fix incorrect reference counting of default context when first created.

* See http://libusbx.1081486.n5.nabble.com/Libusbx-devel-stress-program-crashes-in-libusb-exit-NULL-tp474p477.html
Windows: Add VS2012 solution files

* As well as try to clean up MS's MSVC mess...
Core: Filter out 8-bit characters in libusb_get_string_descriptor_ascii

* Closes #68
Core: Use HAVE_SYS_TYPES_H and HAVE_SIGNAL_H

* These files may not be available on all platforms (eg. WinCE)
Windows: Add windows_common.h header

* This file contains definition that will be shared between the
  Windows and Windows CE backends
Windows: Simplify poll_windows and add provisions for WinCE

* Because poll_windows now requires struct usbi_transfer to be
  defined, it's inclusion in libusbi.h had to be moved down.
WinCE: Add support for WinCE (sources)
WinCE: Add support for WinCE (Solution files)