User-visible changes

Allow use of ECS metadata where available, using new functionality in aws.ec2metadata (>0.1.5). (h/t @jon-mago #23, #30)
Incorporated standard environment variables AWS_SHARED_CREDENTIALS_FILE and AWS_PROFILE into code as appropriate and tweaked locate_credentials() accordingly.
signature_v4_auth() and signature_v2_auth() now returns all inputs to facilitate using the return value in constructing an HTTP request.
Updated documentation.

Bug fixes

Fixed a bug in the default datetime argument in several functions. (#28, h/t @yansonz)
Removed some tests from execution on CRAN.