Add unit tests.

I see appengine.AppEngine before exportAll_() as the real implementation. The export step is just cosmetic. So I think that testing functions like memcacheGet_ makes sense.

I'd like to add tests to confirm that calling new appengine.Appengine().exportAll_().memcache.get does indeed delegate to memcacheGet_, as opposed to some other function. I think I'll do it in a later PR though, this one is big enough as it is.