Implement Window.sessionStorage: Storage Task, Storage Methods (excluding Storage event, QuotaExceededError)
auto merge of #4157 : nkdalmia/servo/master, r=jdm

Changes
* Implemented Storage Task
* Used Storage Task in methods of storage.rs
* Updated webstorage test expectations

Pending Changes:
* Handle Storage Event
* Throw QuotaExceededError in case of failure for method setItem
* localStorage as alias of sessionStorage