Some helper functions for getting cart totals.
Allows you to get the total for a cart (including all recursive items... or not - your decision!)
Allows you to include the tax prices if required (included by default)
Allows you to get only the tax price (including all recursive items... or not - again your decision)

Builds on the pre existing getExtPrice function and adds a new getExtTax function.
Update CartItemInterface to include the default parameters and new getExtTax function.
Fixed tabs instead of spaces
Merge pull request #24 from shipleyr/features/total-helper-functions

Some helper functions for getting cart totals.