Enhance update/create API

[sbrouil]

It could be useful to be able to add custom fields while creating or updating a bug as the WS bugzilla API allows it. At the same time, adding more official fields than just the main ones ("summary", "priority", "product", "component", "version", "status", "resolution", "op_sys", "platform", "severity") seems to be urgent !

[Thorn1089]

Custom fields will definitely be accommodated in some fashion in the released version. As for the enum-valued fields, while the current values are limited, this is intentional -- it's far easier to add new values over time after deliberation and nearly impossible to remove values.