Properly encode URI paths, closes #222

#222

Properly encode the source mapping URL and source URLs array in accordance to RFC3986 as required by the [Source Map spec](https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit#heading=h.75yo6yoyk7x5):

> Note: &lt;url> is a URL as defined in RFC3986; in particular, characters outside the set permitted to appear in URIs must be percent-encoded.

//cc @jmeas