Don't require Ruby/GStreamer

Because it is not portable yet. :<

It only supports only old GStreamer (0.10.0) but old GStreamer isn't
provided by Homebrew.

We can use new GStrmeamer (1.X) by GObject Introspection based
bindings but GObject Introspection isn't work on Windows. :<

[rabbit-shocker:1089]

Reported by znz. Thanks!!!
Support multibyte local file

GitHub: fix #15

Reported by Masafumi Yokoyama. Thanks!!!
Support multibyte relative local file path

GitHub: fix #16

Reported by Masafumi Yokoyama. Thanks!!!
Fix relative local file path
Indent
Merge pull request #17 from myokoym/fix-relative-local-file-path

Fix relative local file path

Patch by Masafumi Yokoyama. Thanks!!!
Add test file of slideshare.rb