Normalize url

This patch implements the net (after conversation) functionality of the following two bullets on the Walker Project Plan

* Ensure we normalize URLs (NOTE: some of these may be a non-issue, just want to check them):
* Configurable parameter exclusion (case insensitive) with defaults: jsessionid, phpsessid, aspsessionid

According to the docs (http://golang.org/pkg/net/url/#Values) it looks like when the values get encoded they will be sorted by key, so we do get that normalization for free (see `Encode()`). Looks like we could simplify this function a lot by just getting `u.Query()` out, deleting the query parameters we don't want, and then setting u.RawQuery = `theValues.Encode()`

One issue, which I think is an existing bug here too, is that the Values map is case sensitive, so we can't just call `Del()` (or do the dictionary lookup). Think we might have to use a case-insensitive regex matcher on the RawQuery and then delete what we find in the way described above, unless you can find a better way.