Add namespace for localStorage keys

[fsmanuel]

Implements a configurable namespace for localStorage keys used by the addon.
For backward compatibility this is a opt-in feature. To activate it there are the following options:

// config/environment.js
module.exports = function() {
  var ENV = {
    modulePrefix: 'my-app',
    'ember-local-storage': {
      namespace: true, // will use the modulePrefix e.g. 'my-app'
      namespace: 'customNamespace', // will use 'customNamespace'
      keyDelimiter: '/' // will use / as a delimiter - default is : 
    }
  }
};

If a namespace is provided (true or custom) the format of a key is ${namespace}${delimiter}${key}.

TODO:

• Add a readme section

closes #168
ref #222

[vst]

Thanks! Looks fine to me... Will try to check on working codebase.