Feat/support additional http methods

[amineo]

Adds support additional methods (post, head, delete)

ExUrlBox.post/3

This action is async. should provide a webhook_url option to receive a postback to once the screenshot render is complete.

ExUrlBox.head/3

If you just want to get the response status/headers without pulling down the full response body.

ExUrlBox.delete/3

Urlbox will cache previously created screenshots for some time. Sending a delete request removes a previously created screenshot from the cache.