Postgres - Implement more of AlterColumnExpression

- modifying column to be nullable or not nullable
- alter default constraint
- delete default constraint

Closes #384
New schema query for DefaultValue

Checks if the a default value (constraint) exists for SqlServer,
Postgres and MySql. Uses the query in an integration test for
MigrationRunner.
Add more tests for Postgres Alter Default Value
Merge pull request #410 from daniellee/issue384-postgresnotnull

Fix alter column for Postgres