Added NSPR support for thread-local storage. Patch submitted by vladimirv. I have slightly modified the patch to still define ANGLE_OS_WIN and ANGLE_OS_POSIX.
BUG=54
Review URL: http://codereview.appspot.com/2497041

git-svn-id: https://angleproject.googlecode.com/svn/trunk@454 736b8ea6-26fd-11df-bfd4-992fa37f6226