Support for metadata extensions for algorithm support.

Closing after release.

Reworked algorithm support registry:
http://svn.middleware.georgetown.edu/view/cpp-xmltooling?view=rev&revision=786

Adjusted digest/signing helpers, and reimplemented encryption helpers with multi-credential support:
http://svn.middleware.georgetown.edu/view/cpp-opensaml2?view=rev&revision=573

Digest/signing method implementation work:
http://svn.middleware.georgetown.edu/view/cpp-opensaml2?view=rev&revision=572

Wired to signing logic:
http://svn.middleware.georgetown.edu/view/cpp-sp?view=rev&revision=3310

xml-sec exposes a method for mapping algorithm URIs into a handler for the algorithm, and throws an exception on unsupported or blacklisted algs. A flag to enable selection of algorithm based on the metadata extension should allow for calling into methods on the RoleDescriptor interface that iterate over the extensions and test for support before returning the algorithm to use.