Marshalize structs

[aounleonardo]

Closes #5

Added two methods, one in encode and one in decode to check if a struct implements the interfaces BinaryMarshaler and BinaryUnmarshaler.

Inside the encoding and decoding methods, apply the function to the structs that implement it.

[ineiti]

LGTM - can you try what happens if you do go test ./... in the cothority using this branch?

[aounleonardo]

[ineiti]

@kc1212 or @jeffallen - merge this if you think it's OK.