Implement Shell sort

[hoytech]

If we add a shell sort implementation I think we'll have pretty much all the "classic" algorithms covered.

I think I have a print-out of Pratt's Shellsort and Sorting Networks somewhere, I'll see if I can dig it up and send a pull request...

[jgamble]

Good idea. Let me know if there's anything I can add.

[jgamble]

Okay, #6 (Make the module object-oriented) is done (bearing in mind there's still clean-up to do).

It's a massive change, so If you have a shell sort implementation done already, I'd just extract the code to a separate file, pull down the new master, and then create a new branch for the shell sort code.

[hoytech]

I haven't got a chance to work on it yet, so no worries!