Fallbacks do not work

[danielweck]

Tested against the EPUB TestSuite 0100 (Reflowable content) and none of the fallback tests pass.

Bugzilla:
https://app.devzing.com/Readium/bugzilla/show_bug.cgi?id=52

[danielweck]

See:
http://www.idpf.org/epub/301/spec/epub-overview.html#sec-access-fallbacks
http://www.idpf.org/epub/301/spec/epub-publications.html#sec-fallback-processing-flow-manifest

[danielweck]

Related issue (parsing):
readium/readium-js#64

Unlike the above readium-js issue, this shared-js issue must result in the implementation of fallback selection heuristics (accessibility criteria, media type support, etc.). The algorithm may require some data input from js-viewer (e.g. user preferences, via viewer settings?).