Renames methods for Span
Changes Span to use a precondition instead of an IsEmpty check
Optimizes span usage in DIT and adds invariant
Changes interval enumerator to use IntervalsEndingInNode instead of the three sets to make searches faster
Removes Pex project files
Fixes plotter to make x axis logarithmic
Adds old implementation of IBS for benchmarking
Merge branch 'master' of https://github.com/lundmikkel/C5

Conflicts:
	C5.Performance.Wpf/Benchmarker.xaml.cs
	C5.Performance.Wpf/Plotter.cs