api.put: use PUT requests, as suggested in API docs, fixes #19

Current docs clearly state:

  **Important**
  On platforms that support the PUT request, PUT should be used to
  upload files to OneDrive when possible to avoid errors that can
  occur when POST is used.

-- http://msdn.microsoft.com/en-us/library/dn659726.aspx

Probably wasn't there before, otherwise can't think why I used POST
in the first place.