Move JavaScript tests to test/js/*

In preparation for adding C tests.
Add C test framework

TODO:
* add newlib toolchain (pnacl is slow...)
* re-enable test timeouts (building is too slow)
* add real C tests
* don't require user to have NaCl SDK installed
Allow running newlib C tests
Building C tests is a test now

This is nicer because if it fails, it will fail the tests. It also
automatically tracks running time, prints errors, and all sorts of other
nice things.
Add npm-debug.log to .gitignore
Use macros to implement interfaces.c

* rename function to use new style nb_*
* use extern "C"
Rename var functions to use new style
Add __cplusplus guard for bool definition
Add fake_interfaces (fake pepper interfaces)

This lets us test via sel_ldr
Add conversion PP_Var <-> JSON
Increase C test timeout

Also, better error message when building C tests fails.