Replace Module Metadata editor with xtext based json editor

This commit replaces the forms driven Module Metadata editor with an xtext
based editor for metadata.json. It also removes all special builders and other
validation code in favor of the building/validation done by xtext.

The change of editor type was motivated by the decision to deprecate the
Modulefile in favor of metadata.json and by the fact that all other editing
in Geppetto is done using text editors.

The deprecation of the Modulefile many new additions to the metadata.json
file are also covered by this commit.

The RMAP is changed to use Eclipse Luna which meanst that xtext 2.6.1 is used.

The following issues are closed by this commit:

GEP-9   - Add support for new metadata properties
GEP-24  - attached file with hashes is corrupted by the formatter
GEP-58  - validate and build Modulefile in nested modules
GEP-102 - Make metadata part of the model used by the linker
GEP-131 - update ghpublisher for Geppetto 4.2 changes
GEP-132 - update geppetto for Puppet 3.6 style metadata.json release builds
GEP-136 - Add Editor support for operatingsystem
GEP-137 - Add Editor support for puppet_ and pe_requirements
GEP-140 - Remove old Module Metadata editor
GEP-141 - Upgrade Geppetto to Eclipse "Luna" release train
GEP-144 - Store checksums in a separate checksums.json file
GEP-481 - Long lines in description destroy the layout of the Modulefile editor