Functions to access the Amazon S3 storage service.

Each function takes a map of credentials as its first argument. The
credentials map should contain an :access-key key and a :secret-key key,
optionally an :endpoint key to denote an AWS endpoint and optionally a :proxy
key to define a HTTP proxy to go through.

The :proxy key must contain keys for :host and :port, and may contain keys
for :user, :password, :domain and :workstation.