Change variable name: resource_path -> resource_iri; use the term of EPUB spec
Use Addressable::URI#route_from to determine Manifest::Item#href

Instead of Pathname#relative_path_from because a bug on Addressable::URI#route_from
was fixed at Addressable 2.3.5.
Add Task#detect_media_type and use it while making EPUB
Downcase extension when detecting media type
Use MimeMagic to detect media type of files
Add changelog about media type detection
Define EUPB::Maker::Error
Keep temporary directory remained on error to help research
Add a changelog about remaining tempdir