Compact Block Relay BIP

The document can be found on Github and includes minor tweaks to wording and clarification that have been made since its creation. Gregory Maxwell, who has been testing the system for a couple of weeks, reports over 96% reduction in block-bytes sent and greatly reduced bandwidth spikes. In opportunistic mode, his measurements show 73% of blocks transferred with 0.5 RTT even without prediction, 87% if up to 4 additional transactions are predicted, and 91% for 30 transactions.