Added support for path API request style.

Rather than using virtual host style requests, path style requests can be used
instead.

i.e. rather than bucketname.s3.amazon.com/... the s3fs will be able to request
from s3.amazon.com/bucketname/...

This is useful for S3 compatible APIs which don't support the virtual host style
request.

It is enabled with the new option, `use_path_style_request`.

Example:

    /usr/bin/s3fs data ~/netcdf -o url="https://swift.rc.nectar.org.au:8888/" -o use_path_request_style -o allow_other -o uid=500 -o gid=500