Add support for grouping signals together

[cilerler]

Group signals together using the alt/else, opt, loop, and par keywords.

Alice->Bob: Authentication Request
alt successful case
Bob->Alice: Authentication Accepted
else some kind of failure
Bob->Alice: Authentication Failure
opt
    loop 1000 times
        Alice->Bob: DNS Attack
    end
end
else Another type of failure
Bob->Alice: Please repeat
end

this should cover #149 and #152

[vin-ni]

Will this be supported anytime soon?