Changes while preparing my Eurucamp talk

- Add a parser to convert HTML to Hexp
- support attribute selectors
Expand the API
Add another alias for set_attributes/%/add_attributes. Might throw some of them out before merging
Change how nodes are frozen to be more performant
API extensions
Make map_children an enumerator
Move all errors in to a single file
This is some of the fallout from my Eurucamp spiking stuff

This commit is not neatly split up, I'm sorry. It contains API improvements,
and it tries to get the metrics back up so we can merge this in.
Bump coverage and metrics so we should be all green
Add to the Changelog for 0.2.0
Add jruby to the "allowed to fail" travis build, returning different YARD coverage metrics than on MRI
Merge pull request #8 from plexus/eurucamp2013

Eurucamp2013