Update invoice.rb

Added Currency Rate to Invoice
Update invoice.rb

Currency rate set to decimal, not string!
Merge pull request #112 from tradegecko/master

Adding Discount Rate to Line Item Record
adding a :logger option in the xero application initialization
Merge pull request #113 from eduvo/needlogs

adding a :logger option in the xero application initialization
Fixed saving as invoices raising "\xE2" from ASCII-8BIT to UTF-8
add user model
add stub response for users api
Merge pull request #115 from davidlumley/feature-add-user-model

Add User Model
Merge pull request #114 from abdulsattar/master

Fixed saving as invoices raising "\xE2" from ASCII-8BIT to UTF-8
Merge pull request #108 from tubedude/master

Adding Currency Rate to invoices