Support async calls

[guilhermesilveira]

I would love to able to do:

f(self, result)
# here i have access to the request result

play(self)
future = Restfulie.at(uri).with(async).callback(f).get()
# future can be polled with future.finished? and future.response
# when the response is received, invoke the callback function

[pmatiello]

Closed with Issue 1.