Add create new store capability

[julienetie]

Due to the way versioning works it is not possible to modify the schema of a database without creating a new version.

But this can be worked around by:

• Copying existing stores from the DB to be modified
• Deleting the database
• Creating a new DB with new and existing store names
• Populating the existing data back to the stores they came from

This API will be in the form of:

• db.addStore()
• db.deleteStore()

Taking one or many store names. And will do nothing if the store exists or does not exist respectively.

This feature will simplify the lifecycle management of Indexeddb.