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
Fix typo in README.
Merge pull request #376 from tomtaylor/master

Fix typo in README.
add podfile
Merge branch 'add-podfile'
Merge branch 'master' into 2.0-development