Add documentation around overriding `initWithDictionary:error:`

Fixes https://github.com/Mantle/Mantle/issues/336
Remove extraneous newline
Update example of pre-computed value from _titleLength to _retrievedAt
Update NSDate property to copy semantics
Remove unnecssary explanation about pre-computed properties
Fix indentation for consistency
Fix convention for checking self in -init
Merge pull request #365