Rename "Fixing Price" to "Exchange Rate" in Currency Exchange Lines ( http://en.wikipedia.org/wiki/Exchange_rate ). Use 6 places for precision in exchange rate.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@17826 20353a03-c40f-0410-a6d1-a30d3c3de9de