Merge pull request #85 from pmienk/develop

Restore structure to targets lost during nonrecursive refactor.
update checkpoint.
check for valid pubkey in check_signature (before openssl would fail sig check but now secp256k1 just asserts).
Accomodate secp256k1 signature breaking change.
Merge pull request #87 from evoskuil/develop

Accommodate secp256k1 signature breaking change.
Allow tests and their dependencies to be optionally included with --with-tests configuration parameter.