Add Adamantium::Mutable

Rationale:

Sometimes an object *must* be mutable. But it can end up in a frozen
object tree. Instead of using `memoize :foo, :freezer => :noop` the
object itself could make sure it does not *really* get frozen.

Use. With. Care.
Add module documentation
Merge pull request #17 from mbj/adamantium-mutable

Add Adamantium::Mutable