new API for floors for tax rates

[Chris Travers <..hidden..>]

Hi all;

I have added a new API (this may change over time) which places floors
in tax rates.  This allows local tax rates to kick in when a specific
amount is above a certain threshold.  The specifics of how this is
handled are module-dependent.  The Simple tax module ignores this
field and so I will probably use CSS to hide it in the default
stylesheet.

Any general feedback here?

Best Wishes,
Chris Travers