Use callbacks to accumulate output in htmljs.

This avoids lots of extra string creation; we accumulate all the output
in a single place from the beginning.
Import packages for Spacebars compiler from meteor and nodify them.
Add tests using spacebars-compiler.
First draft of handlebars to JSON IR compiler.
Merge pull request #2 from cscott/sbqt

Spacebars compiler targetting JSON IR.