v0.4

This is a big update. Added some new features.

• classes MetaSingleton and DataBase was extracted into file database.py
• added driver property for DataBase class
• added processing special chars (like +, / etc) for table parameter
• added type list for table parameter
• added DISTINCT for select() method
• added parameter with_values=True for get_sql() method
• added methods:
  • __str__()
  • union() and union_select() for UNION and UNION ALL in SELECT queries
  • expects() and except_select() for EXCEPT in SELECT queries
  • intersect() and intersect_select() for INTERSECT in SELECT queries
  • has_error() instead get_error() (in the next version)
• written unit test for all SQL methods
• README was separated for some files
• added create_view() method (CREATE VIEW view_name AS SQL_SELECT)
• added drop_view() method (DROP VIEW view_name)