Added support for temporary session credentials...
* importing `com.amazonaws.auth.BasicSessionCredentials`
* new private fn `aws-creds` instantiates creds

Thanks to @marshallbrekka for bringing this up. Closes #21.
Replaced memoized `s3-client` fn with core.cache & core.memoize solution...

Allows multiple clients created using temporary session credentials to be cached
under a single key by providing a :client-id in the `cred` map - Closes #15