Implement JWT

when I changed it to `case object`, I got this error when I try to use it 
```
[error] /Users/skhater/Workspace/Cloud9ers/jwt-scala/src/test/scala/io/really/jwt/JWTSpec.scala:57: type EmptyJWT is not a member of object io.really.jwt.JWTException
[error]     intercept[JWTException.EmptyJWT] {

```