Move converter to Ruby gem

[rjyounes]

The gem can then be shared by the web app and commandline scripts.

Allow different input/outputs, either by different converter classes or by different methods in a single Converter class:
bibid2marc
bibid2marcxml
bibid2bibframe
marc2marcxml
marcxml2bibframe

If the catalog specified in config file doesn't support a marcxml extension (or other uri format) to retrieve the marcxml file, include the marc2marcxml step.
What to do if the catalog doesn't support a url to get the marc? Then can't do the conversion.

Provide a --mode option: batch vs single (i.e., one xml/rdf file with a single , or one per id.