Implement Window.sessionStorage

[jdm]

http://www.whatwg.org/specs/web-apps/current-work/multipage/webstorage.html#webstorage

This can also implement a version of localStorage that aliases sessionStorage, since we don't support any permanent storage in Servo right now. We'll probably need to have a WebStorageTask that watches for requests (either to set/get) for a given URL and accesses the appropriate storage hash based on the origin (probably the eTLD instead, but we can leave a fixme and correct that when we fix #633 as well).

[jdm]

Note: this may be worked on by students from NCSU; if you are not said students, please contact me before starting any work to ensure that no duplicate effort occurs.

[zslayton]

I'd like to try my hand at this. Is it currently being worked on?

[jdm]

@zslayton This task was basically completed, with the exception of dispatching storage events. I'll file a new issue about that in particular.