StoreMap fixes, cleanups and more tests

Remove the mapAtInit = false functionality which fixes #25. Instead provide a remap() function when allowRemap is true that the component using the mixin can call to remap on demand later.

For #24. I think this is really a un-usual use-case where a subclass will override the mappedKeys of another class. But in that case this is now documented that one must concatenate the mappedKeys of the super class and not just replaced the mappedKeys.