Visualize rev tree

Unfortunately implemented #447 here and #440 and some small fixes documented in log:

* fix: get(docId, {rev: some_rev_which_does_not_exits}, ...);
* fix rev option for get in leveldb (returning wrong _rev)

Any comments are welcome as I am still learning how to manage fixing small bugs when writing some feature.

Also: see fixed comments.

Also: I'm not sure if you want to include rev tree visualization as official plugin. Probably not.