v0.1.1: temporary credentials support

New:

• Add a top-level Bucket constructor for every available action
• Support setting a session token to Credentials (using temporary credentials)
• Add RotatingCredentials, a container of Credentials that propagates updates to every cloned copy of it
• Implement parsing responses from the security credentials endpoint of the EC2 instance metadata service

Fixes:

• Fix signing anonymous urls for multipart upload actions
• Improve documentation