Update invoice.rb

Added Currency Rate to Invoice
Update invoice.rb

Currency rate set to decimal, not string!
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
Add in a BadResponse error for HTTP 400/500
- Easier to rescue than a RuntimeError
- Adds a handle_unknown_response_error method for easier overriding
Merge pull request #116 from tradegecko/bad-response

Add in a BadResponse error for HTTP 400/500
Update to the OAuth error message

-scoped old error message to only fire when error_details['oauth_problem'] exists
-added a new catch-all static error message
-with previous approach, you get a "method + not found for nil class" when error_details['oauth_problem'] is nil
Merge pull request #117 from gavinmurphy/oauth-error-message

Update to the OAuth Error Message
Actually name the method correctly for unknown response
Merge pull request #118 from bradleypriest/patch-1

Actually name the method correctly for unknown response