Better concatenation of relative urls (../img.jpg)

Allows "../../logo.jpg" to be properly concatenated with http://example.com/articles/about.html
Before: http://example.com/articles/about.html../../logo.jpg
After: http://example.com/logo.jpg
Merge pull request #7 from rubenrails/better-relative-url-concatenation

Better concatenation of relative urls (../img.jpg)