Initial work
Fix docstring
Clarify that immutability isn't recursive
Optimize some

- try side-step immutable's sentry in with_init
- don't create a new sentry for each instance
Reflow docstring
Merge pull request #15 from hynek/immutable

Add @immutable

Fixes #14
Be mutable by default
Add keyword-aliasing for underscores
Add aliasing examples
Fix doctests
Tweak docs