wsse local options can override globals

This PR should allow local wsse options to override their global equivalents.

Expectations for local overrides are described by https://github.com/savonrb/savon/pull/577#issuecomment-43405260.

A SharedOptions module was created where option logic is the same. As an aside, It might be worthwhile moving the shared soap_header and multipart options there as well, since they are exactly the same (although different comments).