Functions without return statement should return undefined.

Closes #1.
add ~, .actual, .expected to .gitignore
Add interpreter in continuation passing style
Update Makefile and README.

We can now test multiple interpreters.
Factor out util functions.
Strip trailing whitespace.
Add REPL interfaces.
Refactor CPS interpreter to use error continuation.
Rename _this -> thisArg.

`_this` seems to confuse the IcedCoffeeScript compiler.