Gin to Martini, better API handling, users work now

The post handling doesn't work, though. Not in MySQL or SQLite, at least. There
seems to be a weird thing with gorm where it won't actually create foreign keys.
Or even try to handle arrays in databases that don't support them natively. I
haven't actually tested this with Postgres but I assume it'll work there, since
that's what all the demos use. I'll probably end up just ditching gorm (yay...)
and try and find another ORM for Go that actually works properly! This has been
fun... (not)