Allow for policy to be remotely generated.

[timbrandin]

One can now pass the policy as a third argument to the put method, but still keeping the simplicity of this package with support for client side only.

[benjreinhart]

Cool, thanks for this!

Few things before I would consider merging this:

1. My personal style would be to have a simpler method signature, so I wouldn't use the default argument syntax in this case since the default value is a relatively complex expression.
2. Could we add a test for this?
3. Can we update the documentation?

Can you provide an example of how you're using this? Is the policy generated entirely on the server or do you have client side code to pass in some of the attributes of the policy along with some having been generated on the server? Others have been curious about patterns for generating the policy on the server and how that'd work with this library so it'd be awesome if you could provide a detailed example in addition to updating the documentation. That would also give me confidence that this is working correctly since I haven't myself been generating policies on the server.