fallback for old archives with mime-formats
fixed last commit: old archives with mime formats.
added option to change format
added support for description in omex constructors
support for setting the description of omex objects
added support for versioned omex uris
location in the archive now start with a period.

that's been done for compatibility reasons. some other libraries
were not able to read our exports.
started clean up functioanlity
added support for replacing files
implemented test for testModifyMeta
extended test for replacing file