Handlebars.parse/precompile/compile can now be passed an already-compiled Handlebars AST.
Merge pull request #429 from machty/extension-support

Open Handlebars compilation to externally-compiled AST
Decouple compiler version checks from release versions
Better error messages for precompiler runtime mismatch
Switch to mocha (only) for acceptance test
Cleanup and refactor acceptance test
Refactor spec task to run rspec and npm test suite