Added the option to set the database name, version and table name

In response to the [issue #82](https://github.com/mozilla/localForage/issues/82), I've added a **setDbInfos** method for each driver allowing users to set the name and version of the database, and also the store name. There is still some risks using this method, such as changing the store name without updating the database version.

I've also seen the [pull request #99](https://github.com/mozilla/localForage/pull/99), which could offer a good way to set some database informations.

I'm waiting for your comments before writing tests and documentation.