Add ability to specify AWS s3 host through AWS_S3_HOST environment variable
The AWS s3 host used by read_csv currently defaults to s3.amazonaws.com and there is currently no way to change this. This pull requests adds functionality where the code checks the AWS_S3_HOST environment variable for a user-specified host and defaults back to the current s3.amazonaws.com host. 

This is a simple code change, but it will allow users from around the globe to use read_csv to directly read from files in s3 when the s3 bucket is not located in the US East region.
