Fixed and improved SQL exporter
- Dropped use of Strings.SQL_* template strings
- Removed Strings class
- Added SQL __table(), __column(), SQL __primaryKey(), __foreignKey(), __index(), __indexes(), __insert() and __insertField() utility methods
- Added SQL _tables, _indexes and _inserts dictionaries to use in __str__()
- Added SQL __init__() helper to initialize dictionaries
- Updated SQL __str__() formatter for last changes
- Changed index creation to CREATE INDEX statements