Do not return simple functions anymore.

Instead return a MustacheContext object that is characterised
as a lamda
Modified the MustacheContext to act also as a function
Fixed failing MustacheContext tests
More modification to the MustachContext

handle the case of a non lambda object. In that case we just
return the value of the delegate
Modifications to the ExpressionToken

in order to use the MustacheContext as a function
Modified the StartSectionToken to make use of the new MustacheContext