reenable cache

Add require pylibmc, and read env variable to setup memcache.

**Not sure** if there is a **big** improvement with cache (5mb free plan), I went up to ~3.5/4 req/sec vs 2.6/3 with `ab -n 10 -c 10` hitting a single notebook. 

We should apache bench on several url in parallel and try other addons with more cache.