Add support for the s3_force_path_style option for the s3 client

[lancehudson]

This is mostly useful for local development or testing (fake s3 or likewise). This option is needed if subdomains are not mapped (*.s3.lancehudson.com) or if using those domains would cause ssl errors (test.lancehudson.com.s3.amazon.com).

[krobertson]

Thanks!