Coverage

This is a no-brainer, and can be merged safely as it doesn't alter code. In a word:

 * simplify travis yml a bit (by using `before_` and `after_` hooks instead of overriding `install` and `script`
 * prepare to enable nodejs 0.11 (for whenever they fix it and it stops segfaulting on my shoes :-1: )
 * enable code coverage reporting (we have about 80% coverage now, which is quite good IMO!)