Added as_header parameter for oauth1.0_token

Added the the parameter as_header so you can send oauth1.0 parameters in url instead of in header. This is so you can use httr for API that is not using API call’s parameter in the header. The default behavior is not changed.

A small demo is also included for the Withings API to clarify how to use the as_header parameter.
Could you please follow the style guide at http://r-pkgs.had.co.nz/style.html?  i.e. spaces around = and commas at end of line.  Everything else looks great :)