File uploads

This is a branch that should add file uploads to the ruby bindings. The new endpoint is at `uploads.stripe.com/v1/files` which allows users to upload documents to our API and be used in disputes. 

r? @russelldavis (ruby binding maintainer)
cc @jayeb, @amfeng 
Maybe `api_base_url = api_base_url || @api_base` to make it a bit clearer that `@api_base` is the default value.