Enhance filtering

[mtford90]

At the moment filtering is rather basic. If Silk is used for long enough it eventually becomes impossible to narrow down the requests that you want to look at, especially if is running in a busy QA/Production environment. I'm thinking of adding some way in which to "build up" filters as opposed to just having static filters like the current version. The most important will be a way in which to narrow down requests by path/view and time.