mention annotated events support
add forgotten cpu state
refactor grouping logic

* get rid of default_group_by:
  a plugin can't really know how the user wants to have his graphs
  shown.  this 'special' thingie needed too much extra code for no
  obvious benefit.  and the per-plugin-ness conflicts with the idea
  of various metrics from various sources to get rendered on one
  page.   a better future approach might be to allow a user to
  specify some default preferences wrt grouping and displaying.
* provide improved `group by` semantics:
  * introduce concept of soft/hard tags (see readme change)
  * support grouping by an arbitrary amount of tags
  * allow to alter the list of soft tags as well as specify a
    completely new list, overriding the defaults
* change default grouping to group by target_type,what,server. this
  seems fairly sensible
* remove non-obvious hack that caused to always group by plugin
readme update
bugfix: don't match tags target_type/what on graph objects
bugfix: make from/to apply on graphs