Ajax: use anchor tag for parsing urls

This PR changes the Ajax URL parsing from using a regex to using an anchor tag. This should be simpler and more reliable.

Fixes https://github.com/jquery/jquery/issues/1875
/cc @dmethvin
This accommodates some weird IE behavior.
![screen shot 2014-12-11 at 12 37 47 pm](https://cloud.githubusercontent.com/assets/1144197/5400694/98cf1de0-8132-11e4-8221-719c9307c499.png)