OAIExtractor:
* Automatic resumption is working
    - Turn on resumption with --oai-resume
    - Specify resumption token with --oai-resumption-token

Crawler:
* Totally revamped module system
    - Modules are specified together in --crawler-modules and loaded in series
    - Individual classes are be loaded from modules via 'module.class'
    - Data are passed through module chain in order given in configuration
    - Modules don't need to be in the same python module (but they do need to be in
        the same root path for now).
    - TODO: Add modpath for individual modules