Add dependency checker.

lib/guard/yeti.rb

@@ -5,9 +5,15 @@ module Guard
   class Yeti < Guard
 
     autoload :Notifier, 'guard/yeti/notifier'
+    autoload :Dependency, 'guard/yeti/dependency'
 
     def initialize(watches = [], options = {})
       super
+
+      ['yeti', 'node'].each do |command|
+        Dependency.verify_or_raise(command)
+      end
+
       UI.info "Guard::Yeti has started!"
       @latest_output = "sucks"
     end

lib/guard/yeti/dependency.rb

@@ -0,0 +1,12 @@
+module Guard
+  class Yeti
+    class Dependency
+      def self.verify_or_raise(command)
+        output = system("which #{command} >/dev/null")
+        unless $?.to_i == 0
+          raise "Error: #{command} is not installed, which is a requirement of guard-yeti" 
+        end
+      end
+    end
+  end
+end