Fixes for Oracle: Output conversion, "%" in names

Two fixes for the Oracle backend:

1. Support database object names including percent signs. Without this, the backend cannot be used to run the Django test suite, as it fails to create the test database.

2. Use a more efficient method for converting values, esp. numbers, on the way out (code mostly by Ian Kelly, with a small but imported fix after comment by Anssi Kääriäinen)