Use "facets" to resolve extension conflicts

[broofa]

mime-db contains a number of inconsistencies where multiple mime types are defined for the same extension. These should be resolved as part of creating the types.json file, and the define() api should require callers to be explicit about whether or not it's okay to override an existing definition.

$ DEBUG_MIME=1 node
> require('mime')
define() changes "dmg" extension type from application/octet-stream to application/x-apple-diskimage
define() changes "bdoc" extension type from application/bdoc to application/x-bdoc
define() changes "deb" extension type from application/octet-stream to application/x-debian-package
define() changes "iso" extension type from application/octet-stream to application/x-iso9660-image
define() changes "exe" extension type from application/octet-stream to application/x-msdos-program
define() changes "exe" extension type from application/x-msdos-program to application/x-msdownload
define() changes "dll" extension type from application/octet-stream to application/x-msdownload
define() changes "msi" extension type from application/octet-stream to application/x-msdownload
define() changes "wmz" extension type from application/x-ms-wmz to application/x-msmetafile
define() changes "prc" extension type from application/x-mobipocket-ebook to application/x-pilot
define() changes "pdb" extension type from application/vnd.palm to application/x-pilot
define() changes "mp3" extension type from audio/mp3 to audio/mpeg
define() changes "wav" extension type from audio/wav to audio/wave
define() changes "m4a" extension type from audio/mp4 to audio/x-m4a
define() changes "ra" extension type from audio/x-pn-realaudio to audio/x-realaudio
define() changes "wav" extension type from audio/wave to audio/x-wav
define() changes "otf" extension type from application/x-font-otf to font/opentype
define() changes "bmp" extension type from image/bmp to image/x-ms-bmp
define() changes "rtf" extension type from application/rtf to text/rtf
define() changes "sub" extension type from image/vnd.dvb.subtitle to text/vnd.dvb.subtitle
define() changes "xml" extension type from application/xml to text/xml
define() changes "3gpp" extension type from audio/3gpp to video/3gpp