Table Constraints implementation

Story
As a DBA I want to manage table constraints via pgmob so that my productivity improves!

Acceptance Criteria
- Tables should be aware of constraints. This should include:
  - PK
  - FK
  - Unique
  - Not null
  - Check
  - Exclude
- Constraints should appear in table.script() output
- FK should be aware of its dependency and has a reference to it as an attribute
