Apple iBooks metadata parsing (to match existing readium-js behaviour). Partial implementation in the sense that we do not support Sony/Amazon/etc. metadata. This is mostly a bug fix to address the most common case of the "display-options" iBooks XML. https://github.com/readium/readium-sdk/issues/108

Also addressed https://github.com/readium/readium-sdk/issues/121 ("rendition" prefix mapping was missing)

+ minor edits (unused code): erroneous / out-of-date array offsets for map initialisation