* Do not auto-set target bits via autoloader
* Use coin_base to declare target_bits
* Adjust target_bits in each coin_algo class instead
Addresses #1953
* Added coin_base class to extend from
* Defaults in coin_base apply for scrypt/sha256d
* Added coin_scrypt and coin_sha256d that extend without changes
* Updated statistics class to use new coin class calls
Addresses #1953