Support a new API deletePages() to delete existing pages, with REST API support.

Hi Patrick,

Here is another pull request I mentioned in the last one. This request contains the last commit ef937b4, so if you are going to review see 4c1807d for the real diff. (Or if more preferred, I would recreate this request after the last request has resolved.)

This change adds an ability to delete existing (possibly old) pages. This should help to reduce the memory usage of browsermob-proxy, without creating service down time (unlike DELETE /proxy/:port/). Probably no-op for usual testing scenarios, but I think this is especially useful for long-running applications.

Possible scenario for a long-running, incremental consumer: 1) create a new page n+1, 2) get and process a HAR in page n, 3) delete page n, 4) n=n+1 and go back to step 1.

If you think this change is good, could you please take a look at the code?

Thank you,
Hiroaki