A mechanism for configuration of S3 calling formats.

This adds the ability for calling format to be specified when the S3OFS
class is instantiated but doesn't require that the calling class import or
pass in any boto object.

Example:

import ofs
store = ofs.get_impl('s3')(calling_format='OrdinaryCallingFormat')
Merge pull request #7 from NeCTAR-RC/master

A mechanism for configuration of S3 calling formats - thx to @russell.