add clone to Xpath

[claudenobs]

This adds the clone Trait to Xpath so it can be used with the cached macro from the cached crate.

This PR is against 0.6.4 because the master branch currently has some serious performance regressions
(in my case html parsing takes ~2s instead of ~30ms)

[James-LG]

The html module has lots of problems since I hacked it together years ago. I've been working on rewriting it for the past few months which I published to #51 for visibility. It's a huge undertaking so it will take a while.

Your actual change looks perfectly reasonable, but I can't merge it with the out of date branch.

The performance regression comes from these chained replace() calls. Ideally it would do a single pass, but that turns out to be quite complicated, and it would feel like a waste of time given my html module rewrite would throw it all away.

So instead I have attempted to reduce how many times it is called in #52 as a temporary fix until the rewrite is complete. Let me know if the james/perf branch helps at all. If it does, I will merge that, you can update this branch, and then I will merge this PR.