resolve-uri takes a source and target URLs and produces a resulting absolute target-uri

Solves [Issue #8](https://github.com/wtetzner/exploding-fish/issues/8)

Needed to put in a special case that handled "//path" style relative URLs. Tests and implementation based on:

RFC 2396 (more details in this: http://www.ics.uci.edu/~fielding/url/test2.html).