Use Flask-Cache to handle... caching!

Use Flask-Cache [1] to handle caching.

Pros:
* Small dependency that support different backends (redis, memcache etc)
* Handles memoization, could be useful (e.g. for some library providers stuff we discussed previously, or in a general manner for preemptive actions)

Cons:
* One more dependency
* Flask-Cache doesn't handle the fact that your cache service *might* not be available (!) - (naive attempt to fix this: https://github.com/martinfilliau/flask-cache/commit/888348dfdd98e8da810ea757374ee809c26169cf)

Not sure if we would still need the KVService if we were using this.

See [2].

[1] http://packages.python.org/Flask-Cache/
[2] http://stackoverflow.com/questions/11020170/using-flask-extensions-in-flask-blueprints