README: add missing libssl-dev to a list of Debian packages required to build

Conflicts:
	README
make validate_block::check_proof_of_work a public method.
linkage BC_API symbols for validate_block.
blockchain stuff moved to libbitcoin_blockchain.
move logger defines to define.hpp
mmfile move constructor.
remove docstring leveldb ref
README: Added additional Mac OS X build instructions
Merge pull request #37 from kevinejohn/develop

README: Added additional Mac OS X build instructions