Add support for NOT NULL field constraint

Add support for a field to declare a constraint of NOT NULL.
Add support for unique constraint across multiple columns

Add support so a Unique constraint can be created across multiple
columns in a table.
Bump version

Increment version to account for NOT NULL and Unique across
column constraints.
Merge pull request #18 from BrightcoveConsultingReusableComponents/moar-constraints

Moar constraints