Add POST capability

Adds config for method and data attributes, then adjusts EM and Net::HTTP-based checkers to use it. (Note: There's a bugfix lurking in here for Simple too, since it's still using hash notation instead of object. I think this is wrong, since it explodes.) Adds Rakefile-fu for asking for attributes.

Was tested with both EM and Simple checker on Heroku using RequestBin, both existing GET and new POST methods. Works great!

Docs got a bit of love, but might need more.