Adding a script to check for upgrades

tools/check_for_upgrade.sh

+#!/bin/sh
+
+current_epoch=$(($(date +%s) / 60 / 60 / 24))
+
+if [ -f ~/.zsh-update ]
+then
+  source ~/.zsh-update
+  epoch_diff=$(($current_epoch - $LAST_EPOCH))
+  if [ $epoch_diff -gt 6 ]
+  then
+    echo "[Oh My Zsh] Would you like to check for updates?"
+    echo "Type Y to update oh-my-zsh: \c"
+    read line
+    if [ "$line" = Y ] || [ "$line" = y ]
+    then
+      /bin/sh $ZSH/tools/upgrade.sh
+    fi
+
+    # Set the last epoch to the current so that we don't ask for another week
+    echo "LAST_EPOCH=${current_epoch}" > ~/.zsh-update
+  fi
+else
+  # TODO: refactor this so remove duplicates
+  # Create the ~/.zsh-update file with the current epoch info
+  echo "LAST_EPOCH=${current_epoch}" > ~/.zsh-update
+fi
+
+