encode unicode src, test added

Addressing https://github.com/jashkenas/coffee-script/issues/2853,

This change escapes unicode characters in the source code for base64 encoding.
Although the unicode characters themselves are **not recoverable** in the web inspector, any ascii characters will be intact.

After a quick reading of the web inspector [source code](http://goo.gl/cbPE4), I don't think it is possible to safely encode unicode characters for now.

Also added a test that ensures that unicode characters are properly escaped from now on.