Read default region from config file

[samoconnor]

Following from: #34 (comment)

... it doesn't seem quite right to duplicate the region env lookup and default (the other place is here: https://github.com/JuliaCloud/AWSCore.jl/blob/master/src/AWSCore.jl#L109)

There is a slight problem in that struct AWSCredentials does not have a region field so the region from the .config or .credentials file can't currently be accessed by the aws_config function. I propose the following:

• Add a default_region::String field to struct AWSCredentials.
• Set default_region=get(ENV, "AWS_DEFAULT_REGION", "us-east-1") in the default constructor
• In dot_aws_credentials, set default_region if specified by the config files.
• In the aws_config function, change the region= kw parameter to region=creds.default_region.
• I think this should preserve existing behaviour, keep the default in one place, and add support for reading the region from the config files.