move Post->PostForm, add Post method

Post opperations to the chef-server that require data need an io object
The current chef.Post implements the behavior of PostForm from standard
library so I renamed it to chef.PostForm, and added chef.Post. chef.Post
takes an io object and sends it to the server with the signed headders.
add test of the Post method
Merge pull request #3 from spheromak/master

move Post->PostForm, add Post method that takes io.Reader as a body