Fix #1: mismatched response length

[Fimeo]

The last fix seems to pose a problem

In the case of a payload containing several recipients, the line

exponent/client.go

Line 95 in f2c95e8

r.Data[i].MessageItem = msgs[i]

causes an index out of bouds when adding message items in each response object.

Match each MessageItem with the response id.

According to the expo documentation, the output order of push token receipts is guaranteed by following the order of the messages in the input payload.

This merge request offers a solution issue #1 on mismatched response length.

The signature of the push single method had to be adapted to return an array of MessageResponse (one for each recipient of the Message).

Also fixes the NewClient with nil argument to avoid panic on nil pointer exception.

Update readme to match new function signature.

Contains breaking changes on push single method.

Thanks

[9ssi7]

Super except for a small change. Thanks for your contribution