Implement add reference

[mwatts15]

Requirements:

• It should be possible to source every statement added
• Adding a reference should be possible for single statements as well as groups of statements
• References must be tied to unique identifiers for the purposes of querying them
• A group of statements (a graph) with distinct references need not be tied to the same graph name
• Data pulled from either Pubmed or Wormbase should be identified as such in the database.
  • Identification must be on the identifier for the reference.
  • Additional data sources must follow the same structure