Document posible errors on decode and encode functions

[albertoig]

Could be fine to document these errors to handle it better in other developments that use this library. What do you think about it? i don't mind to do it.

[ghost]

When I decoded raise an error of token expired (and yes it is). But I want to get the token decoded, and after, I will check if the token expired or not. I run "jwt-simple": "^0.5.1"