Allow Custom Adjustments to be untaxable.

Added a patch compatible with the latest DEV version.

I think if we're going to roll with this, it should be generalized to a set of checkboxes of all adjustment types, not expect an administrator to type in adjustment names. Adjustment types are all knowable, so generating a complete list of options is not complicated.

Is there a rationale for why someone might ask not to tax shipping, though? That is often locale dependent ... but perhaps in flat rate scenarios the store is just backing the tax amount out of it?