Support gettext 3.0.0

Gettext 3.0.0 removes deprecated APIs.
See also: https://github.com/ruby-gettext/gettext/blob/master/doc/text/news.md#300-2013-08-31

API changes:

require "gettext/tools/poparser" ->
require "gettext/po_parser"

GetText::PoParser ->
GetText::POParser

require "gettext/runtime/mofile" ->
require "gettext/mo"

GetText::MoFile ->
GetText::MO
Support gettext 2.x again for Ruby 1.8

Gettext 3.0.0 drops Ruby 1.8 support but YARD doesn't drop Ruby 1.8
support yet. So YARD supports old gettext that supports Ruby 1.8
again.
Merge pull request #693 from kou/support-gettext-3.0.0

Support gettext 3.0.0