Adds REST api for constraint handling

o REST API for GET, POST, DELETE of constraints
o Adds ConstraintType enum to ConstraintDefinition
o Change: Creating the same constraint twice is a ConstraintViolation
o Adds StmCtx.getConstraints()
o Tests proper TxState handling of the various getConstrains() methods
Merge pull request #762 from boggle/2.0-rest-constraints

Adds REST API for constraint handling