add support for generated columns

src/ColumnMetadata.php

src/Scaffolding/ModificationsImplementation.php

src/Scaffolding/PostgresReflector.php

src/Scaffolding/TableImplementation.php

src/Scaffolding/TablesDefinitions.php

tests/Fixtures/.definition.php

tests/Fixtures/GeneratedModifications.php

tests/Fixtures/GeneratedPrimaryKey.php

tests/Fixtures/GeneratedRow.php

tests/Fixtures/GeneratedTable.php

tests/Fixtures/PackagesTable.php

tests/Fixtures/TestsTable.php

tests/GeneratedTableTest.php

tests/Scaffolding/ScaffoldingTest.phpt

tests/initializeDatabase.php