make configuration immutable

[chadwhitacre]

Right now configuration is half-mutable: you can change attributes on website, but if those settings affected anything in Configurable.__init__ then resetting the attribute will make the configuration inconsistent. On https://github.com/gratipay/aspen-python/issues/472 we're leaning towards making config immutable.

[pjz]

http://pyrsistent.readthedocs.org/en/latest/ might be useful for this

[chadwhitacre]

@Changaco Do you care about this one?

[Changaco]

It's not in my priorities, but making sure that changing the config is either impossible or straightforward is definitely something we want.