Target the new 0.12 spec
Rename 'string' to 'text' to match spec terminology
Allow images to contain images
Changed parseEmphasis to just put things on the stack
Rewrote parseLinkLabel to be consistent with stmd.js and spec
Stack-based link handling
Allow images to contain images
Fix parser not registering blank lines after ATX headers for purposes of tight/loose list calculation

Exmaple:

    * item
    * # block1

      ## block2
Remove distinction btw atx and setext header in AST.

Now we just have 'header' -- Setext and ATX are just two ways
of forming these; it's not a semantic difference that should remain
in the AST.
Merge remote-tracking branch 'origin/spec-0.12'