…forms now work with the same OpenCV version (3.1.0)

- Updated MSVC build script to clone the 3.1.0 branch. Also updated the cmake commandline (3.1.0 needs opencv_ml to be built as well)
- Updated MSVC readme to use 3.1.0
- Updated osx build script to also clone 3.1.0 branch instead of whatever is currently at master
- Due to a bug in OpenCV 3.1.0 it is not possible to call cvGetSize from C source files under MSVC (it will crash due to incorrect code generation). As a workaround for this (until the bug in OpenCV is fixed), the psmove_tracker.c file (which is the only code calling cvGetSize) is now force-compiled as C++. See opencv/opencv#6221 for more info.
- In order to be able to build psmove_tracker.c as C++, some code needed to be updated (extern "C" added where relevant, tracker_default_settings can no longer be initialized using C-style designated initializer)

…all is apparently not compatible with C99. Now using create_tracker_default_settings directly.

- Changed is_valid_float to use std::isnan instead of isnan (isnan is not supported on mingw). std::isnan is the portable version, but is obviously C++ only
- Moved is_valid_float and assert_valid_float macros from psmove_math.h into psmove_quaternion.cpp (this is the only place where it was used anyway) in order to prevent it from being included by .c files by accident (which would cause a compile error due to std:: prefix)

…ctly in psmove_tracker_settings_set_default