task/10011

 - refactored contentmanager
 - added ContentPlugin constructor
 - content types now managed as plugins
 task/10011

 - ensure the :type parameter is lowercase for requests to /api/content/:type
 task/10011

 - added function to iterateSiblings to ContentPlugin
 - added sort order update to article plugin
 task/10011

 - rewrote all schema as json schema
 - updated database interface
 - adapted mongoose dml to new database interface
 - using json-schema-mapper for schema import
 task/10011

 - updated clipboard schema
 - deleted unused contentSchema
 task/10011

 - fixed unset "_type" attribute in content
 task/10011

 - added support for referencing plugin schemas
 task/10011

 - article/block now inherit from contentobject
 - contentobject and children now default to max _sortOrder + 1 on insert