Xaxis label cuts off bugfix

Please check carefully this one as this is my first pull request ever in github. Please let me know of any improvements/problems - I'll be glad to fix it.
The issue: https://github.com/oxyplot/oxyplot/issues/185 

I haven't found a suitable method for rotating the bounding box, so I wrote it myself.
MeasureText now accepts `double angle = 0`, that controls the angle of text measured. 

I have also added an example of other issue mentioned in the ticket. It is quite near the original issue, but the code responsible for it in in the other place, I yet haven't looked at that. 

Tests are green, everything except Xamarin compiles - I wasn't able to compile Xamarin because of license, asked a question in gitter. ~~But it should work as the change in xamarin is very simple.~~ I will use buildserver to check that for now
I think it should be a separate example. To reuse code, you could create a private method that returns the `PlotModel` and give the angle as parameter. Also, it would be interesting to see if it works for the different axis positions (left, right, top, bottom). I suggest to create plots that has axes on all 4 sides, and set the same angle to all of them.