Would like to be able to serialize int64 as string

[mylanconnolly]

Hello all,

I am working on a project that I'd like to utilize jsonapi for.

I'm running into a snag because our ID format is large int64s (like 1362085645204177151). These overflow when just serializing to JSON (the last few digits end up being just zeros), so when using just plain JSON serialization in Go, I'd do something like:

type Example struct {
  SomeID int64 `json:"someId,string"` // The string argument tells the json encoder to format this as a string
}

This doesn't seem to be implemented in this package, and I'd not be opposed to doing the legwork myself, but I just wanted to see if this is something that would be approved before I got too far.

Thanks!

[shwoodard]

Pull requests welcome.

[mylanconnolly]

Created pull request #121. Thanks!