Added documentation for invoice creation

Added documentation for invoice creation which shows how a customer can be linked to the invoice without making an extra round trip to xero.
Added save_all method to BaseModel.
Heh. Whoops.
Fixed reference to OAuthCredentials in the acceptance test.
Merge upstream/master into fimmtiu/add-bulk-operations.
Added tzinfo to dependencies to fix test errors.
Moved logic into the mark_dirty/clean methods.
Merge branch 'master' into add-bulk-operations
Added 'ref' as a dependency to the gem.
Added documentation for the save_all method.
Re-adding mistakenly removed require line.
Added 'summarizeErrors=false' flag to PUT & POST requests.
Fixed bug where unmodified records could be saved.
Fixed possible nil reference with GCed objects.
Prevent newly built records from being marked as dirty.
Fixed indentation in oauth.rb.
Added logging for raw HTTP traffic.
Merge branch 'log-http-traffic' into add-bulk-operations
Revert indentation changes.
Revert indentation changes.