Precompiled headers

[boberfly]

What I've noticed from building Cortex over and over is that it takes way too long! What I propose is having a Precompiled.h (or stdafx.h) and place includes that are commonly used through the library to speed up compile times.
MSVC:
http://msdn.microsoft.com/en-us/library/h552b3ca.aspx
GCC:
https://gcc.gnu.org/onlinedocs/gcc/Precompiled-Headers.html

Thoughts? Bringing in the Export.h took awhile and I'm somewhat reluctant to add another include into every file again, but the compile times at least on MSVC on my laptop are quite long.

[boberfly]

Adding this link commit as it's exactly what Pixar's USD has done for MSVC support:
PixarAnimationStudios/OpenUSD@791bdb6