Non-forking extra nonce added to Bitcoin header.

Block version set to 3.
Unittest included (in src/test/miner_tests.cpp).
Re-define 15 unused bits of the version field as an extra nonce inside the block header.
Accompanied by a BIP, see wiki (https://github.com/BlockheaderNonce2/bitcoin/wiki).
Not forking.
Backwards compatible with GBT clients that are not aware of this.

Timo Hanke & Sergio Lerner