i18n: make gettext gem optional

Require gettext related files lazy and ignore LoadError.

lib/yard/i18n/locale.rb

@@ -1,6 +1,3 @@
-require "gettext/tools/poparser"
-require "gettext/runtime/mofile"
-
 module YARD
   module I18n
     # +Locale+ is a unit of translation. It has {#name} and a set of
@@ -31,6 +28,13 @@ def load(locale_directory)
         po_file = File.join(locale_directory, "#{@name}.po")
         return false unless File.exist?(po_file)
 
+        begin
+          require "gettext/tools/poparser"
+          require "gettext/runtime/mofile"
+        rescue LoadError
+          return false
+        end
+
         parser = GetText::PoParser.new
         parser.report_warning = false
         data = GetText::MoFile.new

spec/i18n/locale_spec.rb

@@ -21,7 +21,13 @@ def locale(name)
       @locale.load('foo').should == false
     end
 
-    it "should return true for existent PO" do
+    have_gettext_gem = true
+    begin
+      require "gettext/tools/poparser"
+    rescue LoadError
+      have_gettext_gem = false
+    end
+    it "should return true for existent PO", :if => have_gettext_gem do
       data = <<-eop
 msgid ""
 msgstr ""