Relay first double-spend as alert, and notify

Relay at most one double-spend per previous transaction output, subject to anti-DoS provisions.

Unconfirmed double-spends carry information that is time-critical to the payee of the first spend, but today, they are silently dropped.

Add a -respendnotify option that can immediately notify a user-supplied command when a double-spend affecting the wallet is received.

Change the GUI wallet to highlight a transaction in red when a transaction that double-spends it has been seen.  When the conflict first occurs, display a warning dialog.

![alertshot](https://cloud.githubusercontent.com/assets/4284124/3654442/eb871ada-1169-11e4-97e6-1dd049d1c15f.png)

This change has been running on mainnet since 2014-05-02.  Relayable double-spends seen by the change are [recorded here](http://54.186.244.42:5984/respends/_design/explore/_list/list1/date%2Ctime%2Chash?descending=true&limit=40) in realtime.