Added SQLServer specific unicode insert.
Merge branch 'master' into pr378
Move ExplicitUnicodeString to core namespace

 - Otherwise, the user has to introduce an extra using:
Fluentmigrator.Model which is unneccesary.
Merge pull request #378 from Ashthos/master
Readding check if transaction exists to firebird's rollback override
Implement Reverse function for CreateConstraintExpression
Don't exclude firebird from CanCreateUniqueConstraint() test
Integration tests for autoreverse CreateConstraint

 - With and without schema
Closes #379 pull request from concept-hf/firebird-tpmreview