Tests are run with locally installed coffee

This is good at least for two reasons:
- we can be certain that the tests are run with the `coffee` version specified in [package.json](https://github.com/mileskin/bacon.js/blob/9dccf55a19e7b9be27348af5addf9ac15a82f190/package.json#L22)
- tests can be run even if `coffee` is not installed globally

While at it, all shell scripts now use the same shebang line. See http://en.wikipedia.org/wiki/Shebang_(Unix)#Portability