Add opentxs-core library

Same as https://github.com/monetas/opentxs-server/pull/32 for this.

Now you should be able to chose the path where your opentxs is with
-DCMAKE_PREFIX_PATH=/home/lucas/tmp/opentxs/

Can somebody test that on his system too?

To install opentxs in another directory than the system default (/usr/lib) do
cmake .. -DCMAKE_INSTALL_PREFIX=/home/your/prefix/