Autoload the parsers on first use

Adding this gem to an application should probably load the gettext
parsing utilities when it's being used at runtime.  Currently, the
gem entry lib file requires parser which eager loads these parsers
and consumes time and memory when the developer or system may not
need them.

lib/gettext_i18n_rails_js/parser.rb

@@ -24,11 +24,10 @@
 # WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 #
 
-require_relative "parser/base"
-require_relative "parser/javascript"
-require_relative "parser/handlebars"
-
 module GettextI18nRailsJs
   module Parser
+    autoload(:Base, "gettext_i18n_rails_js/parser/base")
+    autoload(:Javascript, "gettext_i18n_rails_js/parser/javascript")
+    autoload(:Handlebars, "gettext_i18n_rails_js/parser/handlebars")
   end
 end