Added tests for PostgresGenerator

 - Create Primary Key constraint
Postgres - Implement more of AlterColumnExpression

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

Closes #384
Adds Support For Indexes With Include Columns

Includes tests to verify behaviour.
Merge pull request #409 from matthewdunsdon/CreateIndexIncludeSupport

Adds Support For Indexes With Include Columns using SqlServerExtensions. 

Closes #339 and #336.
Fix exception message
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.
SqlServer Alter Column Default Value fix

When altering a column's default value and setting it one of the
SystemMethods it used to returned the name of the enum rather than look
up the correct function. Closes #398 and closes #394.
Merge pull request #411 from daniellee/issue398-AlterColumnSystemMethods

SqlServer Alter Column Default Value fix
Add more tests for Postgres Alter Default Value
Merge pull request #410 from daniellee/issue384-postgresnotnull

Fix alter column for Postgres