feat(driver): add basic chrome.storage.local driver

[vikrantpogula]

I've written a basic driver for using chrome.storage.local inside Chrome Packaged Apps. It works pretty well for my application but it's kind of hacky in places.

Just putting it here so that others can try it / find issues and help me in writing tests and making this good enough for inclusion.

requested in #43

chrome storage API : https://developer.chrome.com/extensions/storage

[vikrantpogula]

for anyone else looking into it the main issue here when compared with localStorage / localStorageWrapper is that the chrome API ( chrome.storage.local ) seems to do some kind of internal serialize/deserialize.

I will work on this further and try to implement proper serialization for all data types

[tofumatt]

I've thought about this more and it's out of scope for localForage to include in its base functionality, but thanks to our custom driver API you could release it as its own module and get users to extend localForage with it.

Sorry for the hassle of you going through a pull request for it, but I think you should release this as your own library and add it to the Wiki. I've created a page for custom drivers and will link to it from the README (#325)