support tagging bins

[rodneyrehm]

In a DM @remy mentioned to me that jsbin/jsbin had a (deep, hidden, private) API to tag bins. Providing the ability to set tags in POST /api/save (and POST /api/$bin/save) and read them from GET /api would allow us to remove the constraint of having to map source files to bins in the source file via <link rel="jsbin" href="…">. After talking to @jzaefferer this may be a prerequisite for pushing the jQueryUI examples to jsbin.com - discussed in jquery/jqueryui.com#96.

The first step to figuring out how to tackle this, is to get some info from @remy regarding the private tagging API.