Support IAM roles and environment variables for credentials

[larowlan]

[deviantintegral]

This fails if you run tests on servers that aren't EC2 (like local environments).

PHPUnit 4.6.6 by Sebastian Bergmann and contributors.

Configuration read from /Users/andrew/vagrant/d7/www/docroot/sites/all/modules/amazons3/phpunit.xml

...............F................................................. 65 / 69 ( 94%)
....

Time: 1.51 seconds, Memory: 53.50Mb

There was 1 failure:

1) Drupal\amazons3\S3ClientTest::testFactoryEnvironmentIAMGetCredentials
Failed asserting that exception of type "\Aws\Common\Exception\InstanceProfileCredentialsException" is thrown.

FAILURES!
Tests: 69, Assertions: 124, Failures: 1.