builtin sorted function and reference implementation of sort making it stable.

Adds a reference implementation of the python `list.sort` method and of the builtin `sorted` method. Addressing #19,  and re-iterating #43.

I ran `dist m`. 

I need to wrap the sorted function with `native.js`. And try out all the permutations of the parameters. But when I add `Sk.nativejs.func(...)` to `sorted` it blows up because it's undefined at that moment. @rixner do you know why?