TransactionModel UnitTests

[PieterCK]

Added new helper functions to quickly create a number of randomly populated journal entry, ledger, account and transactions in DjangoLedgerTest .
Added unit tests for:

• TransactionModelForm
• TransactionModelFormSet
• get_transactionmodel_formset_class()

[elarroba]

Not sure this is is the right approach for writing unit tests.
I'd like to see more tests geared towards testing the application logic and not the core Django functionality itself.

For instance, instead of testing if formsets are working the way they're intended (which is a Django-spefific test), we should be focusing on the TransactionModel business logic itself.

For example:
- Testing if amounts could be saved as negative transactions (which they shouldn't).
- Testing if a TransactionModel can be modified when Journal Entry is locked.
- Role and Account validity for certain situations.
- If testing forms, will a Formset allow the user to modify a transaction from a locked journal entry?
- If locked journal entry, are the form fields properly greyed out in the UI to show that transactions can't be modified.

These are just examples of business logic that needs unit tests.
I will gladly accept your contribution and modify it accordingly.