Upload templates to S3

[andrewdmay]

Although it's possible to do this with the upload method, if there are nested stacks it's necessary to upload multiple templates.

Could be an option in the configuration - e.g. a TemplateBucket option, and then scan the template for nested references, potentially replacing relative paths with S3 URLs before uploading the root template (this process needs to be recursive as there can be multiple levels of nesting).