Clear local storage data upon new version of AS

#### To test:
 - npm install
 - grunt build
 - Run your web app with recently compiled version of AS and make sure that local storage has new  AS_version key that matches with its compiled version from package.json. 
@mmontenegro @lsvx @maxicecilia,  I've created this new internal logic for AS storage.  So that we don't  worries anymore about of its context.  Now we just call for Storage.set( key, value) and storage module will do the rest. 

As you can see right before (https://github.com/natgeo/modules-activitystream/pull/38/files#diff-95c0fd6f52480dfbe077f9821b5836c6L40)  we were setting the prefix "AS/" for each time we want to save a special key for AS. Now it is more consistent.