Add javascript example for truncated signature

[koenpunt]

Since some of the other examples also demonstrate the use truncated signatures, I've added it for the javascript example.

[DarthSim]

Hi,

Thanks for your contribution! Could you put the truncated signature example in a separate file so it won't puzzle ones who don't need truncation?

[koenpunt]

Could you put the truncated signature example in a separate file

Ha, I saw a similar comment on the PHP version after I submitted this PR, so I was already expecting this feedback. Although I think in this case it's a bit different, since it's just a simple slice on the digest value, whereas with the PHP version there's a new method call introduced.

But if you really want to me to split it I can do that as well.

[DarthSim]

I like examples to be copy-paste ready with as less redundant code as possible. So let's split it :)

[koenpunt]

Took me some time, but I updated this PR to have the example in a separate file.