Use GetExecutingAssembly() instead of Environment.CurrentDirecotory

Closes #907
Update src/Nancy.Hosting.Self/FileSystemRootPathProvider.cs

Change from null check to not-null as per Andres' suggestion.  Also compare local assembly var instead of calling GetEntryAssembly 2x
Merge pull request #909 from btompkins/FixRootPathProvider

Use GetExecutingAssembly() instead of Environment.CurrentDirectory