gtk3: add Gtk::Window#set_titlebar (enable the use of Gtk::HeaderBar)
Merge pull request #236 from tescorg/master

gtk3: add Gtk::Window#set_titlebar (enable the use of Gtk::HeaderBar)

Patch by tescorg. Thanks!!!
travis: use the latest Ruby 2.1
poppler: install Poppler with --with-glib option for Homebrew

GitHub: fix #237

Reported by Colin Dean. Thanks!!!
clutter-gstreamer: remove needless note
clutter-gstreamer: show text on control

Because a segmentation fault has been fixed.
clutter-gstreamer: call Gst.init
clutter-gstreamer: remove needless note
clutter-gstreamer: remove needless note
Add Ruby 2.1 because it is already supported
gtk3: use spaces for indent

    % inplace "sed -e 's/\t/        /g'" *.rb
gtk3: remove trailing spaces

    % inplace "sed -e 's/ *$//g'" *.rb
gtk3: use " (double quote) for string literal
gtk3: use Cairo instead of Gdk::Drawable

Because it is GTK+ 3 style.
gtk3: scale drawing area to the window size
gtk3: indent
Fix broken links on GitHub
travis: add a TODO comment about clutter-gst-2.0
gtk3: add a sample for Theming/Style Classes
poppler homebrew: remove needless `--with-glib` option

Now, it is the default.

GitHub: rabbit-shocker/rabbit#24

Reported by Kazuhiro NISHIYAMA. Thanks!!!