order total plugins

[DerManoMann]

Hi there,
Converting order total plugins is on the list of things for 0.9.9 and I've been pondering that for a while now. A order total plugin is just a regular plugin, but implements additional interfaces (this is a change from 0.9.8). My idea is to have a single method (more for credit class), that returns:
- name - the text displayed
- one or more of:
* amount
* tax group id
The point is that the code calling the order total methods can build subtotals for each tax class and tax needs to be calculated only once for each tax group. This should improve rounding behaviour a bit (I hope).
For credit class there are more methods to be called, however the main call for the ot line is the same.

Opinions - suggestions - ideas?