Introduce 'scrollalble' option, make 'dataSource' an explicit option, include usage hints in Readme

This is a rather big pull request and maybe it should be split up. Most is not so hard to read, but in the spec I have both re-indented and added lots of tests, which looks a bit bad in the diff.

Otherwise I have introduced the ```scrollable``` option, which is explained in the newly added readme and also discussed in https://github.com/sunesimonsen/knockout.list/issues/11. Tests and an example page for scrollable have been added.

For easier usage of ```dataSource```, I have made it an explicit option, while up to now it was only available when passing an object with an 'onUpdate' property to knockout.list. This also allowed to get rid of 'grid.js' file again, which was introduced in my previous pull request.