Fix cache in python>=3.3 (hashing)

Since Python3.3 the ``hash()`` function is randomized by default (you can disable it by env variable but i think its not recommended :) ). The effect is - after server reload/restart old cache files will not be used because hash() value is different.

I've made use of ``webassets.cache.make_md5`` method for creating hash values and renamed ``__hash__`` methods to ``id`` as internal "caching keys" providers. Tests are passing on 2.7 & 3.3 :)