Added support for custom fields.
Custom fields just like custom timers, but they may contain any kind of data (and this is not analyzed, only logged).
In the transaction object, simply create a dict field called 'custom_fields'.
Fixed a bug in the parsing of the results - now we distinguish better between custom fields and custom timers.
Each configuration file can now hold arguments for at test, which are python arguments that are eval-ed, then joined to a dict passed to the run method of each transaction.
In addition, now logging the entire traceback of each error, rather than just the error itself (useful for debugging).