Initializing $.store twice adds duplicate en/decoders

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Running the following code:

```
s = new $.store();
s.set('test', {"a":1});
console.log('Get 1', s.get('test'));
s = new $.store();
console.log('Get 2', s.get('test'));
```

results in

```
Get 1 Object
a: 1
SyntaxError: Unexpected token o
```

This happens because .encoders and .decoders are part of the prototype, and thus globally modified every time `new $.store` is called.

This is an issue because I am using this in a single page application, and use it in just one model where it's needed (instead of storing a global reference and only initializing once).

Start with the $.store constructor and inspect how the prototype-level .encoders and .decoders are initialized; reproduce the provided two-instance example first. Done means creating a second store does not duplicate registrations, and both get calls complete without the reported SyntaxError.

Written by the indexing model from the issue text.