scriptnum unit test
split off script_number into its own header.
migrate script code to use script_number (dropping openssl-dependent big_number)
remove cruft
Merge branch 'develop' of github.com:libbitcoin/libbitcoin into develop

Conflicts:
	test/base58.cpp
Merge pull request #9 from evoskuil/develop

Implement deprecated (msvc), fix test (msvc), explicit casts, EC updates.