Support for AWS_PROFILE override, similar to the php sdk. Override applies only for the default profile, not for specifically named profiles.

Fixes issue #249 by adding environment variable profile override support for the Java SDK.

This [amazon security blog post](http://blogs.aws.amazon.com/security/post/Tx3D6U6WSFGOK2H/A-New-and-Standardized-Way-to-Manage-Credentials-in-the-AWS-SDKs) mentions the ability the override the default profiles using the AWS_PROFILE environment variable, but it doesn't look like the java sdk ever implemented it.