Adding support for providing the token via header.

This is useful for XHR requests from the JavaScript to the server, where you typically send JSON instead of form encoded data.

Ruby on Rails has a built-in forgery protection similar to ring-anti-forgery, and also allows for setting the token via a header (x-csrf-token), so this is a proven method.
Adding mention of passing token via header in README.
Adding XHR header example to README.

This procedure is also stolen from Rails.
Merge branch 'token-via-header' of git://github.com/augustl/ring-anti-forgery

Conflicts:
	src/ring/middleware/anti_forgery.clj

Change http header for tokens to x-anti-forgery-token

Add test for sending tokens via http headers

Bump version to 0.3.1