Allow passing file as the payload body with em-http-request

The feature is described in:
https://github.com/igrigorik/em-http-request/wiki/Streaming#streaming-data-off-disk

# Streaming data off disk

> EventMachine provides a send_file like utility for efficient uploads of files directly off disk – all you have to do is give EventMachine the name of the file and it will do the rest.

``` ruby
EventMachine.run do
  http = EventMachine::HttpRequest.new('http://www.website.com/').post :file => 'largefile.txt'
  http.callback { |chunk| puts "Upload finished!" }
end
```