Implement destroy(), use global db store

Fixes #22.

There's now a global store that keeps track of all memdown databases, keyed by location. so if you do:

```js
var db1 = new MemDOWN('foo')
var db2 = new MemDOWN('foo')
```

then both `db1` and `db2` will contain the same data.

I also added a destroy test here, rather than in abstract-leveldown, because memdown isn't even passing the latest abstract-leveldown tests. If you want to see these tests pass at 100%, we're going to need to peg abstract-leveldown to v0.12.3 due to the snapshot test.