Add 'UNITED STATES' to the list of countries using US Dollars.

United States was missing from the list using USD currency.
Merge pull request #20 from hso/master

Add 'UNITED STATES' to the list of countries using US Dollars.  With a facepalm for missing the obvious; thanks to hso for the pull request.
Support for abs operation ( __abs__ )
Merge pull request #21 from toudi/master

Support for absolute value of Money instances: abs operation (__abs__)
Merge remote-tracking branch 'upstream/master'