Change installation script

Instead of asking the user to clone the repo and running rake
install, there's is now an easy .sh script that can be downloaded
and invoked in a single line.

The script takes a parameter "ask" if the use prefers to control
which modules of yadr are installed.

install.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+if [ ! -d "$HOME/.yadr" ]; then
+    echo "Installing YADR for the first time"
+    git clone https://github.com/skwp/dotfiles.git "$HOME/.yadr"
+    cd "$HOME/.yadr"
+    [ "$1" == "ask" ] && export ASK="true"
+    rake install
+else
+    echo "YADR is already installed"
+fi