Only store embedding & basic meta data for Face Instance when call toJSON

[huan]

Store imageData separate, for better performance when calculating distance() for many cached faces.

[Scapal]

I would rather put the distance method on embedding and also add a serialization method on it rather than putting it only on the face class.

[huan]

@scapal good idea.

Would you like to send a pull request to show your idea?

[Scapal]

@zixia Yes I may have some time by the end of the week to do so.