Compressed pubkeys

This patch enabled compressed pubkeys when -compressedpubkeys is passed.
These are 33 bytes instead of 65, and require only marginally more CPU
power when verifying. Compressed pubkeys have a different corresponding
address, so it is determined at generation. When -compressedpubkeys is
given, all newly generated addresses will use a compressed key, while
older/other addresses keep using normal keys. Unpatched clients will
relay and verify these transactions.