srf.formats.tagcloud(), add tests and update to tagcanvas 2.1.2

* Move TagCloud class into SRF namespace
* Update to Tagcanvas 2.1.2 (06/05/2013) [1]
* Enable async loading
* Conditional RL loading
* Make docs compatible with JSDuck
* Eliminate excanvas support and instead output an error msg

Less code, same functionalty, added test

[1] http://www.goat1000.com/tagcanvas.php#links

Change-Id: I4d3d26e8bdb6e78bcca4f0fb79fda7e2bf6cdaa5