Add support for generic value transformation in mappings

[kattrali]

Right now OR supports value transformation for dates in a common format, or (implicitly) dates in other formats by overriding defaultFormatter in NSManagedObject subclasses.

It would be convenient to support a more generic kind of value transformation. Two ideas:

Option 1 - Formatter Classes

Add a mappings property for formatter that takes an NSFormatter subclass as a value. When a formatter is present, the value for that key is processed through -[FormatterInstance getObjectValue:forString:errorDescription:] before updating the record. Here's an example usage:

+ (NSDictionary *)mappings
{
    return @{
             @"id" : PRIMARY_KEY,
             @"sandwich" : @{
                     @"class": [Sandwich class]
                     @"formatter": [SandwichFormatter class] },
             @"timestamp" : @{
                     @"formatter": [SuperSpecialAPIDateFormatter class] }
             };
}

A caveat would be that we'd presumably need to store any instances of formatters in thread dictionaries, as they may not be threadsafe.

Option 2 - Inline Blocks

Add a transform block to mappings, for inline value formatting. Example:

+ (NSDictionary *)mappings
{
    return @{
             @"id" : PRIMARY_KEY,
             @"sandwich" : @{
                     @"class": [Sandwich class]
                     @"transform": ^(id value) { 
                           return [SandwichFormatter sanitizeParams:value] } },
             @"timestamp" : @{
                     @"transform": ^(id value) { 
                           return [[SuperSpecialAPIDateFormatter sharedFormatter] dateForString:value];  } 
             }
             };
}

Managing thread safety would be up to the user (and therefore NMP), though this way would probably have more code duplication.

[supermarin]

Closing this in favor of #66