Make the README use local assets, not remotely-hosted ones.

When Near took down byuu.org, he also removed the images that the bsnes
README was ointing to. He gave me backups of data from the site, including the
images used on the bsnes homepage, but apparently the README used different
images hosted in a different location.

I've replaced the missing images with ones from the bsnes homepage that
look reasonable.

README.md

@@ -1,7 +1,7 @@
 bsnes
 =====
 
-![bsnes logo](https://byuu.org/images/bsnes/github/byuu-bsnes-logo.png)
+![bsnes logo](.assets/logo.png)
 
 bsnes is a multi-platform Super Nintendo (Super Famicom) emulator, originally
 developed by [byuu](https://byuu.org/about), which focuses on performance,
@@ -70,6 +70,6 @@ Nightly Builds
 Preview
 -------
 
-![bsnes user interface](https://byuu.org/images/bsnes/github/byuu-bsnes-user-interface.png)
-![bsnes running Bahamut Lagoon](https://byuu.org/images/bsnes/github/byuu-bsnes-bahamut-lagoon.png)
-![bsnes running Tengai Makyou Zero](https://byuu.org/images/bsnes/github/byuu-bsnes-tengai-makyou-zero.png)
+![bsnes user interface](.assets/user-interface.png)
+![bsnes running Bahamut Lagoon](.assets/bahamut-lagoon.png)
+![bsnes running Tengai Makyou Zero](.assets/tengai-makyou-zero.png)