Credentials need to be thread-safe

[mholt]

Issue #13 addressed the thread safety of signing requests, but highlighted that now the credentials are potential hazards for race conditions.

[peterbourgon]

😎

[mholt]

@peterbourgon My coworker Dane is working on PR #16 which should fix this. All of the Sign functions become variadic, and will accept their own instance of a Credentials struct to use in addition to a request. If credentials are not provided, then that function will make its own instance using environment variables or the IAM role. There will be no more shared Keys variable.