Initial novacoin support

* New satoshi-per-coin constant value;
* New address versions;
* Fee calculation adjustment;
* Fix pubkey outputs spending issue;
* Change titles from bitcoin to novacoin;
* Handle coinstake and coinstake transactions properly;
* Add scrypt hash function for headers verification;
* Remove block header bits verification for now;
* Remove exchange ratio, coinbase and other bitcoin related plugins;
Rename files