Check if Zsh is installed prior to installing Oh My Zsh

Check if Zsh is installed before installing anything else.
New-commers, or people who don't read the disclaimers(ex. me) will go
straight to stack overflow if chsh -s returns an error.

tools/install.sh

@@ -1,5 +1,12 @@
 set -e
 
+CHECK_ZSH_INSTALLED=$(grep /zsh$ /etc/shells | wc -l)
+if [ ! $CHECK_ZSH_INSTALLED -ge 1 ]; then
+  echo "\033[0;33m Zsh is not installed!\033[0m Please install zsh first!"
+  exit
+fi
+unset CHECK_ZSH_INSTALLED
+
 if [ ! -n "$ZSH" ]; then
   ZSH=~/.oh-my-zsh
 fi