Minor tweaks

Noticed multiple identical calls to parseUri(url) in a couple of methods... it's only a small change but seems it  would be more efficient to parse the url once and then re-use the returned object.

Also added `resrc.configure({/* options */})` as a shorthand to `resrc.extend(resrc.options,{/*options*/})` to make my code look nicer :)

Haven't included tests as changes are small and obvious - plus the contributing guidelines don't specify a testing framework or folder to put them in...