Fileset load method and tests

Added the "method" option to px:fileset-load as requested in [issue 275](https://code.google.com/p/daisy-pipeline/issues/detail?id=275).

Also added a bunch of XProc tests; primarily for px:fileset-load, but also some for px:fileset-filter and px:mediatype-detect. I discovered and squashed several bugs in these modules while writing the tests as well.