Json marshal implementation

[semihbkgr]

It is the implementation of json marshaling that preserves the order of records as maintained by the map. This was discussed in #12, but I needed this functionality for my use case, so I went ahead and implemented it.

@elliotchance, if you still think it's unnecessary to support this in the library itself, feel free to close it or keep it as a reference for anyone who might need something similar.

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This change is 

[elliotchance]

#12 explains why it's not reasonable for orderedmap to implement JSON. There are multiple ways that suit different use cases. I'm sure this worked for you, but someone else will say they need the encoding format to be different. That's why it can't be handled directly by this type.

I'm open to discussing different encoding strategies (like a new type).