sessionStorage support

[iamdtang]

Recently I found myself wanting to use sessionStorage instead of localStorage. What do you think about having this feature? I'd be happy to implement if you think it is a good addition.

[kurko]

Perhaps that should be another lib, ember-sessionstorage-adapter?

[iamdtang]

I thought about that too. I wasn't sure and wanted to see what everyone thinks. It's a small addition to this addon since it'd be simply extending the current local storage adapter class and overriding a few methods.

[locks]

hhmm… it's something to take under consideration. @skaterdav85 would you be willing to make a proof of concept PR?