Change the logo in splash page

[konifar]

Overview (Required)

• Currently the title logo in splash page is just DroidKaigi 2016 text.
• I want to change it to the logo image as same as web.

I had already added img_logo, but it have white background color and black text. We need to create new image file with the transparent background and white text.

[konifar]

This is the image on the web. https://droidkaigi.github.io/2017/images/2017-textlogo.png