Add simple mechanism for clearing all rewrite rules

I thought I'd have a go at adding a mechanism to clear any pending rewrite rules, resolving lightbody/browsermob-proxy#6.

It allows a caller (via Java or REST API) to clear all of the rewrite rules currently in effect, thus allowing new rules to be set up. It doesn't go as far as the per-rule deletion discussed in #6, but I hope that it's useful on its own.

I've had to do a similar thing for blacklist and whitelist for my own tests - there's a pull request coming for those too soon. This one was a easier to submit first!

I'm happy to make any changes requested to fit in with existing code. Just let me know.