Added content editing methods to the graph object

Updated editing-modals plugin to use the new methods
Added the ability to remove a concept

Updated editing modals to use the new method when deleting a concept
Added tests for removing a concept
Updated addContent in knowledge map API
Added tests for content editing in knowledge map
Updated test for removing concepts
Merge pull request #61 from knowledge-map/kg-content-editing

Content editing methods in knowledge map object