Added node handler for partials and simple partial helper.

Partials work! Let me know what there is to change.
  - Partial node handler pushed partial token to our html parser.
  - Partial token is aliased to partial helper in hydration opcodes.
  - Added helper object to env in rendered templates
  - Added helper object to env in html_compiler tests.
  - Added registerHelper function to html_compiler tests.
  - Fleshed out tests for both htmlbars style partials and helper style partials

Also, a comment and an apology – My last commit, the fix to have the compiler ignore comment nodes, was done under a git user whose email I no longer have access to. I changed the commit's email in the log hoping it would be a transparent change, but apparently it shows up. I'm sorry if this causes any problems while merging...