Renamed modules

The core parser modules have no dependencies to Catmandu modules, so
they don't have to be in the Catmandu:: namespace. Catmandu::PICA should
be an empty module that only combines all modules needed to work with
PICA data within the Catmandu framework.
added check for leader
deleted leader from some records
first draft of PICA writer
implemented basic PICA exporter (no XML yet)
Merge branch 'devel' of github.com:gbv/Catmandu-PICA into devel
implemented draft of PICA XML writer
Simplify internal PICA format

and drop of record leader because usage is unclear
Simplify internal PICA format
removed regex modifier
Merge pull request #3 from jorol/issue2

This closes issue #2. For customization of PICA::Writer::Plus I created issue #4.
added Plain PICA parser
use xml as default import and export type