client ssl connection added.

I added the functionality to establish a ssl-connection as client.
This requires a SSL_CTX and must be set by the user.

There might be a better way to integrate it in the library but this should do right now.

NOTE:
It might be wise to add the workaround mentioned at http://www.wangafu.net/~nickm/libevent-book/Ref6a_advanced_bufferevents.html

    SSL *ctx = bufferevent_openssl_get_ssl(bev);

    /*
     * SSL_RECEIVED_SHUTDOWN tells SSL_shutdown to act as if we had already
     * received a close notify from the other end.  SSL_shutdown will then
     * send the final close notify in reply.  The other end will receive the
     * close notify and send theirs.  By this time, we will have already
     * closed the socket and the other end's real close notify will never be
     * received.  In effect, both sides will think that they have completed a
     * clean shutdown and keep their sessions valid.  This strategy will fail
     * if the socket is not ready for writing, in which case this hack will
     * lead to an unclean shutdown and lost session on the other end.
     */
    SSL_set_shutdown(ctx, SSL_RECEIVED_SHUTDOWN);
    SSL_shutdown(ctx);
    bufferevent_free(bev);