Added Memory View to Dashboard, with temp backend.

* Added memory view to dashboard.
* Backend temporarily sends memory used, in bytes, to be changed.
* Added "label" for heat map tooltip.
* Refactor data fetching: now cards are fetched once, and each view is fetched on demand.
* I broke the "Loading" indicator, should redo it robustly.

Signed-off-by: debuger <debuger@softwarefrontier.com>
Merge branch 'master' of https://github.com/dzimine/collectd3