Fix no implicit conversion of nul into String error

  - Fix /usr/local/bin/crew:67:in `+': no implicit conversion of nil into
    String (TypeError) with 'crew build', 'crew install' and 'crew install'
    commands if no package is provided.
  - Fix notice 'Package  isn't installed.' with 'crew remove' if no package is provided.

crew

@@ -534,20 +534,36 @@ when "whatprovides"
     puts "Usage: crew whatprovides [pattern]"
   end
 when "download"
+  if @pkgName
     search @pkgName
     download
+  else
+    puts "Usage: crew download [package]"
+  end
 when "update"
   update
 when "upgrade"
   upgrade
 when "install"
+  if @pkgName
     search @pkgName
     resolve_dependencies_and_install
+  else
+    puts "Usage: crew install [package]"
+  end
 when "build"
+  if @pkgName
     search @pkgName
     resolve_dependencies_and_build
+  else
+    puts "Usage: crew build [package]"
+  end
 when "remove"
+  if @pkgName
     remove @pkgName
+  else
+    puts "Usage: crew remove [package]"
+  end
 when nil
   puts "Chromebrew, version 0.4.2"
   puts "Usage: crew [command] [package]"