JPM/HM: Added addTriple, removeTriple
HM/JPM: fixed getStatements
TS/JPM: fixed 'removeTriple' (with new test + some cleaning up)
TS/JPM: adapted to seecr.test.integrationtestcase
TS/JPM: added guards (Uri checks) to getStatements + some refactorings (API incompatible)
JJ/TS: improved Uri and Literal

- Uri does not inherit from str anymore
- ValueError raised if non-stringlike value given
- Improved test coverage
JJ/TS: spacewhite
JJ/HM: Export/backup function
JJ/HM: Removed prints