Added a facility to catch certain data inconsistencies at runtime e.g. missing data, empty strings.
Merge branch 'RenderContextBehaviour' of https://github.com/mdevilliers/Nustache into RenderContextBehaviour

Conflicts:
	Nustache.Core.Tests/RenderContext_Behaviour_Support.cs
	Nustache.Core/Nustache.Core.csproj
	Nustache.Core/RenderContext.cs