Added filters to the rates in the usps shipping

Added two filters for both International and Domestic shipping rates so plugins can hook them and apply any additional fees, etc. without altering the WP-e-Commerce core code.
Fixed filter prefix of proposed rate filters

Fixed the proposed filter prefixes and added white-space as requested.
Merge pull request #338 from amereservant/patch-2

Added filters to the rates in the usps shipping