Added some additional environment variables to check - the ones commonly
referred to in commandline documentation.

This is pretty much how the Java SDK works (though it does not have the
additional fallback on AMAZON_ that was already there).
Merge branch 'default_env_vars' of https://github.com/friedmag/aws-sdk-core-ruby into friedmag-default_env_vars
Refactored the code for specifying the region and credentials via ENV.