Custom model pathes

Hey,
I have performed some minor changes, so models can be loaded from any path - not just from the install path of pymad, like this:

    # search models also in 'models' subdirectory of current directory:
    cpymad.listModels.modelpathes.append(os.path.join('.', 'models'))

It seems to be working fine. On the other hand, I don't know for what `pkgutil.get_data` and `pkg_resources.resource_string` were used in `cern.cpymad.model._get_file_content()` - so there might be a bit of an redundancy or inconsistency now.

Tell me what you think about this.

Best regards,
Thomas