Add dictionary representation to Objc models

[maicki]

As we have a need internally at Pinterest for a dictionary representation I took a stab on #53. Resolves #51.

@matelo Did a great job lining up all the pieces to get this integration in, I think it's really close to get in, unfortunately it seems he's pretty busy, so hope we can push that over the finish line soonish.

Some changes to matelo's

• Fixes error
• Revert some not needed things
• Update some code styles
• Initialize NSNumbers with Literals

@rahul-malik We should think of also adding some tests that are testing our generated models. Maybe somehow in the integration tests. I thought around something like this:

NSDictionary *imageModelDictionary = @{
    @"height" : @(12),
    @"width" : @(11),
    @"url" : @"http://google.com"
};
Image *image = [[Image alloc] initWithModelDictionary:imageModelDictionary];
NSCAssert([imageModelDictionary isEqualToDictionary:[image dictionaryRepresentation]], @"Should be the same");


NSDictionary *userModelDictionary = @{
    @"id" : @(123),
    @"first_name" : @"Michael",
    @"last_name" : @"Schneider",
    @"image" : imageModelDictionary,
};
User *user = [[User alloc] initWithModelDictionary:userModelDictionary];
NSCAssert([userModelDictionary isEqualToDictionary:[user dictionaryRepresentation]], @"Should be the same");

var imageModelDictionary: [AnyHashable: Any] = ["height": (12), "width": (11), "url": "http://google.com"]
var image = Image(modelDictionary: imageModelDictionary)
XCTAssert(imageModelDictionary.isEqual(to: image.dictionaryRepresentation), "Image dictionary representation should be the same as the model dictionary.")

var userModelDictionary: [AnyHashable: Any] = ["id": (123), "first_name": "Michael", "last_name": "Schneider", "image": imageModelDictionary]
var user = User(modelDictionary: userModelDictionary)

XCTAssert(userModelDictionary.isEqual(to: user.dictionaryRepresentation), "User dictionary representation should be the same as the model dictionary")

[ghost]

	2 Warnings
⚠️	This is a big PR, please consider splitting it up to ease code review.
⚠️	Any source code changes should have an entry in CHANGELOG.md or have #trivial in their title.

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