Allow NSManagedObject formatter to be changed

[vnavarro]

• changed defaultFormatter to formatter
• added a sharedFormatter getter
• exposed setFormatter as public
• added two tests to verify date formatting when
  using setFormatter

Suggestion: Maybe is also a good idea to add some way of changing each date formatting while mapping.

[viking2009]

what about name convenience?

- (void)setDateFormat:(NSString *)dateFormat {
    [sharedFormatter setDateFormat:dateFormat];
}

[vnavarro]

Thanks @viking2009 I think it's much better now.

[supermarin]

@vnavarro thanks for your work!

I do have a few concerns before merging this tho:

• Format should be changeable per entity, so Person +dateFormat and Message +dateFormat could be different
• Not sure if this interferes with @kattrali's Support generic value transformation in mappings #66 . @stephencelis any thoughts on proceeding on this one?

[stephencelis]

Format should be changeable per entity, so Person +dateFormat and Message +dateFormat could be different.

Hm, should it be by entity? I think a global setting actually works well here (as most APIs will use a global date format). For granularity we can fall back to the transformation support defined in #66.

[supermarin]

I've thought @kattrali said a certain API returns different date formats on a few places

[stephencelis]

@supermarin In those cases, I think a transformation could solve the problem. An exception instead of a rule.

[kattrali]

@supermarin @stephencelis Agreed that transformation could handle those special cases. It seems reasonable to assume that different date formats per model would be the exception in most API synchronization flows.

[supermarin]

Sounds good. The last one - should we just expose the whole readonly -dateFormatter so people can change stuff like timezone?

[stephencelis]

@supermarin Hm, so maybe the date formatter should go directly on the CoreDataManager class?