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.


I like the idea of only using the AWS_PROFILE env var if a user hasn't explicitly specified one in the constructor.  This change seems pretty close to that behavior, except for the edge case where a user has explicitly passed "default" into the constructor.  

Could you tweak the code to account for that case?  I don't think we'd want the AWS_PROFILE env var to be used in any case where a user did pass in an explicit profile name.  This behavior would also keep the handling of the AWS_PROFILE env var consistent with the other AWS SDKs.