Update sharedInstance type

[brandons]

sharedInstance is type MutableDictionaryPreferences, however the definition does not specify it's mutability.

This particularly causes an issue when using Alamofire-Prephirences because the associated extensions only work with MutablePreferencesType.

[phimage]

We can have a not mutable shared instance, you can set it, that's why
I know using not mutable preferences is not the general case
I must think about it (if I merge or not)

maybe I can add (but not clean)

public static var sharedMutableInstance: MutablePreferencesType? {
  get {
     return Prephirences.sharedInstance as? MutablePreferencesType
  }
  set {
       if let pref = newValue {
                Prephirences.sharedInstance = pref
      }
    }
}

Prephirences.sharedMutableInstance?.loadFrom...

to not to have to do each time

(Prephirences.sharedInstance as? MutablePreferencesType)?.loadFrom