Merge pull request #283 from benma/cleanup

Removed unused SaveContractWallet(std::ofstream&).
CMake: add OpenSSL version check

On OSX the version check helps to identify if the system OpenSSL
(which is old) is found instead of Homebrew's OpenSSL.
Before, the compilation failed while now we fail while executing
CMake with a much more readable error message.
Merge pull request #284 from toxeus/openSSLversion

CMake: add OpenSSL version check
OTCryptoOpenSSL: move functions into caller
OTCryptoOpenSSL: fix signature of callback definition
Merge pull request #285 from toxeus/OTCryptoOpenSSLCleanup

OTCryptoOpenSSL: cleanup declarations
Made OTContract::GetIdentifier(OTString&) non-virtual.
Removed unused Base64 functions in OTCrypto.
Added -Woverloaded-virtual to PEDANTIC_CXX_FLAGS