A unified way to call the Link#url
Fix last commit
Document Link#url and its overriding methods
Link#end_html doesn't take link_resolver anymore

This parameter was used only to make it look like
start_html
Link's html rendering methods now use #url
DocumentLink's html rendering methods now use Link's ones
Merge pull request #26 from rudyrigot/url_params

A unified way to call the Link#url