fixes typo with JOURNAL_STATUS for ManualJournal
send all create requests using a POST request (untested, could break, ymmv etc...)
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.
Fix ordering
Merge pull request #67 from latentflip/fix-ordering

Fix ordering
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.
Made Xeroizer::Records serializable to JSON.
Merge pull request #70 from fimmtiu/json-for-records

Made Xeroizer::Records Serializable to JSON.
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.