Switch back to using PUT when creating records

Xero doesn't use PUT the way it's defined in REST, but when we switched to using POST for creates it caused some issues for us at Harvest.

See http://developer.xero.com/documentation/getting-started/http-requests-and-responses/

> The HTTP PUT and POST methods are used for sending information to the API.
>
> A PUT method will create new data in Xero, whereas a POST will either create new data or update existing data in Xero.

By using POST, it's possible to overwrite invoices that may have the same invoice number.  For example, say I have an existing invoice in Xero with invoice number "123".  If I try to PUT a new invoice with that same invoice number, Xero responds with a validation error.  If I POST a new invoice with that same invoice number it will "update" the existing invoice with the new data.

I know there was an issue related to the PUT method being used for creates: https://github.com/waynerobinson/xeroizer/issues/32, but it seems a bit heavy handed to change ALL creates in Xeroizer to use POST instead of PUT.  Do we still have the ManualJournal / OAuth issue?  I know you had commented in that Issue that it may be a problem with the OAuth gem:

> I'm not sure what is going on here and haven't had a chance to do my own testing on this at the moment. I know that XeroGateway is having similar problems so there might be a change to the OAuth gem that has broken this.

/cc @zmoazeni