Local Storage accessor and modifier helper
my two cents : 


add .idea to your .gitignore. If someone else uses the same IDE as you and works on this code, it could get very frustrating.
your code is not minification-proof, which can be annoying for someone who plans to use it. See angular's doc (section "a note on minification") for directions on how to fix this.
you dynamically extend String.prototype - this kind of monkey-patching can create conflicts with other libs, and lead to unexpected behaviors. If you want people to use your code, you should avoid this, especially for such simple code that you only use once.
you have two different entities defined in the same file, but that's ok as the whole lib is very lightweight. For more complex projects, split up your code, one entity per file.
your code lacks tests, which is a major no-go for many people.
instead of the quirky ng-change directive, you can directly set a change handler during the linking phase using $scope.$watch('something', function(){...}).