Change uint to float64 for TimeStamps

[MaxBosse]

Hey,

currently the develop branch will throw the following error when parsing a PRESENCE_UPDATE containing a Game object:
2017/10/30 14:52:48 [DG0] wsapi.go:477:onEvent() error unmarshalling PRESENCE_UPDATE event, json: cannot unmarshal number 1509371565406.0 into Go struct field TimeStamps.start of type uint

This change will fix the error by changing the TimeStamps.start and TimeStamps.end values from an uint to a float64.

[njhanley]

Why use a float64 when the payload is an int64?
https://discordapp.com/developers/docs/topics/rich-presence#updating-presence-update-presence-payload-fields

[iopred]

Breaking the API is not the right way to fix this, writing a custom UnmarshalJSON seems right here.

[njhanley]

My previous comment was incorrect in assuming the Discord Rich Presence SDK payload was reflective of the Discord API. I see now that Discord returns the "timestamps" object's fields as integers encoded as floats.

I agree that a custom UnmarshalJSON makes sense but the API still must be changed as a uint is 32-bit on some platforms. I've created a pull request (#475) with the changes I believe are necessary.