Add safepoints, which for now are for GIL checking

Once the GRWL is added, will also be for GC safepoints.
make format
Proactively save the thread registers when calling AllowThreads

And then use that register info rather than sending a signal to the
thread; this lets the thread that called AllowThreads avoid receiving
signals ex during a syscall.

I'm not sure if this is valid though; are we really guaranteed that
the thread can't invalidate the saved state?
Start working on the GRWL (aka gil-rwlock)
merge from origin
Merge branch 'master' of github.com:dropbox/pyston