Convert LocalSetting.default to a read/write property

And remove LocalSetting.get_default()
Don't raise NoDefaultError from LocalSetting.has_default

That should only be raised if someone attempts to *use* the default.
Simplify check to see if default is set
Convert LocalSetting.value to a r/w property

It now falls back to using the default if the default is set.
Validate when setting LocalSetting.value
Remove unused import
Start adding tests

At 34% coverage
Add initial README