Feature make localForage browserifyable

This PR makes localForage browserify friendly. 

- It adds the promise dependency to the `package.json` file as it is required in `src/localforage.js`.
- It replaces `this` by `window` object at function calls because `this` would have the wrong context when browserified.
- It modifies the value of the `browser` field in `package.json` to point to the correct browserifyable `localstorage.js` file.

All tests pass on: 

```
v0.10.31
1.4.23
```