Add support for Cake.Net

Cake.Net creates a tools folder at the root which contains downloaded packages and should not be checked in.

VisualStudio.gitignore

@@ -263,3 +263,6 @@ paket-files/
 # Python Tools for Visual Studio (PTVS)
 __pycache__/
 *.pyc
+
+# Cake
+tools/