Support pyformat and format paramstyle

Currently only paramstyle 'qmark' is supported. This makes it hard to migrate application which use the pyformat style (which is default in MySQLDB and MySQL Connector/C).

Suggestion: Additionally support the following paramstyles:

1. format

cursor.execute("SELECT a FROM b WHERE id=%s", ('foo',))

2. pyformat

cursor.execute("SELECT a FROM b WHERE id=%(name)s", {'name' : 'foo'}))

mixing paramstyles shouldn't be allowed and should result in a parsing error.