Build: add paket.sh file for simpler usage

paket.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+if test "$OS" = "Windows_NT"
+then
+  # use .Net
+  [ ! -f tools/paket/paket.exe ] && tools/paket/paket.bootstrapper.exe
+  tools/paket/paket.exe $@
+else
+  # use mono
+  [ ! -f tools/paket/paket.exe ] && mono --runtime=v4.0 tools/paket/paket.bootstrapper.exe
+  mono --runtime=v4.0 tools/paket/paket.exe $@
+fi