Added support for multiple model types to shared.js

[ldeboer]

Allows shared.js to store cache based on type of data, so it can store multiple objects with the same $pk, as long as they are of different types. Most of this code came from @jasonayre in issue #208. Lines 39-41 were added by me to fix a bug where model scopes would get nested after being cached. This way the scope is refreshed every time it is read from the cache.

[jpulec]

@ldeboer This PR is quite old and out of date. Do you still have any interest in getting it included?