Introducing helper.coffee

This file will be the base for small common services/resources etc. which can be used by any component and not adding any new dependency.
Currently it only has baseurl as a common place to generate the base url which will be used by many components
Standalone page uses baseurl from  helper
Stream page uses baseurl from helper
H main controller uses baseurl from Helper
Merge pull request #1075 from hypothesis/1066-introduce-helper-2

Merging this. Closes #1066