Add user interface to set dust limit and filtered addresses

This patch adds a new "Advanced" pane to the options box where the dust limit for accepted transactions can be set, and where a filter on output addresses can be specified. CTxMemPool::accept() will drop any transaction with outputs below the dust limit, or with an output address corresponding to an address specified in the filter.

![screenshot](https://f.cloud.github.com/assets/420734/273459/ef1212e4-9024-11e2-957a-31f3f8996172.png)