feat(localforage): add support for multiple storage instances

Related to #245, #45 and #4.

**Update**: [here is a fiddle ](http://jsfiddle.net/e1bpzjgd/1/) with the running example.
If I replace the catch part with `throw e;` then `grunt test` prints `"Error: SECURITY_ERR: DOM Exception 18"`, when the two new test cases are run for the WebSQL driver.