added API specification tests for nanopub
added API specification tests for nanopub

tests were added to check
- 400 on invalid BNJ
- 422 on invalid bel_statement syntax
- 422 on invalid bel_statement semantics
- 422 on invalid experiment context annotations

refs #127

OpenBEL/openbel-server,OpenBEL/openbel-api,OpenBEL/openbel-server,OpenBEL/openbel-api,OpenBEL/openbel-api,OpenBEL/openbel-server,OpenBEL/openbel-api