Change cache to a Map and expose it

[VitorLuizC]

After used vuex-cache code for a while I realize it's methods is very similar to Map methods and doing some researches I also confirmed that Maps are better than pure objects(Object.create(null)).

• Objects and Maps compared - MDN
• Performance comparison - JSPerf

So I'd changed cache constant to a Map instance, added dispatch method and expose it. Now all Map methods and dispatch is available under store.cache namespace. Also I'd changed the unit tests and README.

In fact, now this lib is faster and smaller, and the only side effects is IE11- that needs Map polyfill to run properly.