Xaxis label cuts off bugfix2

Original pull request: https://github.com/oxyplot/oxyplot/pull/297

I have created new pull request here to clear the history a bit - the old one is completely reverted.
This one includes the fixes proposed by @objorke .
The example with wrong label placement appeared a bit more complex - so I made it interactive to be able to view all possible angles, instead of predefined few examples. I think it deserves a separate issue, and I think I have a fix for that, but I need a separate discussion, - I'll create a ticket

I have also `OxySizeExtensions` separate file, not sure whether this is a good idea, but I could not find a better place for `GetBounds` method.