Better db isolation control

[tdivis]

• reworked internal implementation of how (or rather when) database creation, transaction begin/rollback, flush and fixtures loading are made so that minimum of these time-consuming operations are made, and developer now have better control when those operations are (or are not) done.
• database isolation control now works for packages and modules too (i.e. if you want db flush after each TestCase or test function in the module, you just put database_flush = True at the beginning of the module)
• added tests for testing features mentioned above