ModelCataloguePlugin #162 create Asset extendible element for file uploads

created Asset element
ModelCataloguePlugin #162 create Asset extendible element for file uploads

simple upload action
ModelCataloguePlugin #162 create Asset extendible element for file uploads

upload action creating new asset
ModelCataloguePlugin #162 create Asset extendible element for file uploads

upload action updates existing asset if available
ModelCataloguePlugin #162 create Asset extendible element for file uploads

frontend for upload
ModelCataloguePlugin #162 create Asset extendible element for file uploads

first fully working example
ModelCataloguePlugin #162 create Asset extendible element for file uploads

extracted storage service
ModelCataloguePlugin #162 create Asset extendible element for file uploads

fixes #162
Merge pull request #178 from MetadataRegistry/issue/162-create-asset-element

Issue/162 create asset element