Peer verification using environment variables

OpenSSL can use the environment variables SSL_CERT_DIR, SSL_CERT_FILE for loading of peer verification files instead of manually loading them via certificate files or verify paths.  Boost asio provides a facility for this via context->set_default_verify_paths, which cpp-netlib must call when not providing the verfication file or directory.  

Also, options.hpp does not provide a setter method for always_verify_peer, so it cannot be changed to true.

Tested on Windows, should work on *nix.