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.

Can this be [configured through boto](http://boto3.readthedocs.org/en/latest/guide/configuration.html#configuration-files)? I think `~/.aws/credentials` can have a `region` key. It'd be nice to use their config system rather than trying to work around in pandas.
 Unfortunately, it can't. You can't add it as an option to the .boto configuration either. [Other packages](https://github.com/jschneier/django-storages/issues/28) have implemented a similar work-around. 
 @mlurie ok, thanks for this

see #12206 for adding more docs w.r.t. s3 and options that we support
