StaticContentConventionsBuilder uses a static Dictionary<string, responseFunc>.

This can lead to unexpected behaviour when there are two Nancy engines in the same AppDomain that differ by rootfolder but use the same path (e.g. index.html) being served from two directories.

Key on a new object that is both root and Path.
Merge branch 'SupportingTwoStaticDirectoryConventionsInOneAppDomain'