Support STS session tokens

[ipmb]

For local development it may be desirable to use STS to retrieve temporary credentials, but it doesn't work currently because the view doesn't pass the token into the signed request as far as I can tell. There is code to add it here:

django-s3-upload/s3upload/utils.py

Lines 52 to 53 in 1f6bfc4

	if token:
	policy_dict["conditions"].append({"x-amz-security-token": token})

Some info on using these to sign requests is here.

To resolve this, a new optional setting named AWS_SESSION_TOKEN could be added that will be passed into the signature generation function as the token keyword arg.

[ipmb]

It looks like this is what's needed bradleyg#170