Plugin reorganization

I don't know how you feel about this, but I find it a bit redundant that most plugins are named something like `hg.ext.lowercasename.CamelCaseNamePlugin`. I think it would be a bit better design-wise, if plugins were grouped by their functionality, so it'd be more like `hg.ext.<function>.SpecificPlugin`. This would also help with discoverability of what a plugin might do.

As such, I've re-arranged the plugins so that they are grouped in the following manner:

 * css:
  * LessCSS
  * Stylus
 * images:
  * ImageSizer
  * JPEGOptim
  * OptiPNG
 * js:
  * Uglify
 * meta:
  * Meta
  * AutoExtend
  * Tagger
  * Sorter
  * Grouper
 * structure:
  * Flattener
  * Combine
  * Paginator
 * text:
  * Blockdown
  * Markings
  * Reference
  * Syntext
  * Textlinks
 * vcs:
  * GitDates

Now, these are not necessarily the best groupings or names of groups, so I'm open to suggestions.