Split sql generation into standalone class

SQL generation is an implementation of RepositorySupportDefinition
Independent pluggable RepositorySupportDefinition implementations are allowed
to support different SQL engines, i.e. PostreSQL, Oracle, MS SQL.
Prettify API a little

Instead of

    StorableType<Integer> column1 = TableColumn.integer("column1").storable();

You should now write

    StorableType<Integer> column1 = StorableType.forColumn("column1").integer().build();

Latter variant is more discoverable and allows to avoid TableColumn class altogether,
wich makes it easier to learn API.
Remove redundant imports