Add `halt` method.

Exit the current block, halts any further processing
of the request, and returns the specified response.
Update README with Halting section
Accept any kind of object as body param for halt function.
Set headers to the halt response.

Now halt receives an options hash where we can set the body, the headers or both.
Use named parameters
Update halting section of README
Merge pull request #14 from emancu/add_halt_method

Add halt method