Get default S3 credentials from DefaultAWSCredentialsProviderChain

As mentioned in the commit message, this fixes seratch/AWScala#11

Very simple - it just uses the DefaultAWSCredentialsProviderChain when calling awscala.s3.S3()

The original usage of specifying credentials with environment variables still works fine, just need a slightly different environment variable name for the secret to match Amazon conventions.
Oops, sorry. My mistake.