(much) faster testing
after priming the cache, the extensions and cythoning will only recompile if files
change. otherwise they are retrieved from the cache.

the caching is disabled by default just in case it's buggy, but i've had no problems
so far.

with caching enabled and using tox_prll.shl it now takes 25 seconds to test all
2.x and about a minute and a half to test everything.

looks good. thanks
