Add stubs to some exercises

[leoyvens]

Add stubs to leap, raindrops, bob, beer song, proverb difference of squares and sum of multiples.

[ijanos]

Can we modify Travis to check type check these? To approve this PR with a good conscience I would have to check every single type signature by hand.

Probably will open an issue/pull request if I got the time, but it is likely that someone will be faster than me.

[leoyvens]

I copy pasted all signatures from the example solutions so it's unlikely that they are incorrect, but I understand your concern. To make the manual review less painful here are the links to the examples:

beer song
bob
difference of squares
leap
proverb
raindrops
sum of multiples

[leoyvens]

This would be particularly useful to me if it could be merged before August 12 when I will be using Exercism at a Rust workshop. I'd really appreciate if someone managed to review before that!

[ijanos]

Merging as this received no objections from anyone in a months time. Sorry for not making the merge before the 12th, I hope the workshop went well. Thank you for your contribution.