Add images in coin and dice module

[swapagarwal]

As pointed out in #114,
Instead of displaying text, we can show an image or a gif of a coin showing the result (heads/tails).
This can be used in the dice roll as well.
See the attachment template. Use the Image type.

[mahimg]

I was trying this out and could get this kind of output,
{"text": "heads", "quick_replies": [{"payload": "{"entities": null, "intent": "coin"}", "attachment": {"attachment": {"type": "Image", "payload": {"url": "http://coins.silvercoinstoday.com/wp-content/uploads/2010/10/America-the-Beautiful-Silver-Coin-Obverse.jpg"}}}, "content_type": "text", "title": "Flip again!"}]}

Is this fine, I am finding it difficult to test it on messenger, need some help with figuring out ACCESS_KEY.

[swapagarwal]

@mahim97, this is being worked on in #141