Add function for reporting the tor version in tor_api.h

As a user of libtor_api it would be useful for me to be able to obtain the version of tor without having to call it's main function.

The function could look something like:

const char * tor_version()
{
    return tor_version_string;
}

This would be very useful for a user of the library that wishes to check if they have linked to the right tor version (or implement some sanity checks) without having to start tor and then speak to it on the control port (therefore paying some significant overhead).