First quick implementation of visualizing rev tree

Usage: db.visualizeRevTree('key', function(svg) { });
Probably will be refactored to function(err, svg);
TODO:
* show each revision data
* add/delete capabilities
Improve visualization

* mark deleted nodes with grey circle (isDeleted broken?)
* currently works only with idb!
Rewrite visualization to support public API

* add open_revs to http adapter
* now it works only with http adapter as open_revs=all is not yet implemented in any other adapter
* new usage: db.visualizeRevTree(docId, function(err, svg){});
Mark deleted, winning and leaf revisions

* deleted = grey stroke
* leaf = green
* winning = red
Implement open_revs using idb adapter, small fixes

* pouch.js - add generic error for bad request
* pouch.merge.js - fix Tree comment
* pouch.utils.js - add comments
* add test for open_revs
* add test for get with rev parameter (leveldb fails!)
Implement open_revs, fix get with rev (leveldb)

* get(docId, {rev: rev}) fixed in leveldb (tests added)
* implement open_revs in leveldb
Fix get a doc with non-existent rev
Implement open_revs in websql
Remove console.logs
Make visualizeRevTree standalone plugin
Fix basic lint problems
Merge pull request #450 from neojski/visualize-rev-tree

Visualize rev tree