Dynamic attribs

Ok, so I've added this as a module *Tire::Model::DynamicPersistence* which needs to be included after persistence, ie:

    include Tire::Model::Persistence
    include Tire::Model::DynamicPersistence

Readme, tests, & updated gemspec included.

In addition to tests for the new functionality, I've also included all the persistence_model_tests from Tire to ensure no legacy behaviour breaks.  I did have to change one test (see inline comments) but behaviour is still compatible. https://github.com/davekinkead/tire-contrib/blob/master/test/dynamic_persistence/model_persistence_test.rb#L261

Cheers,

Dave