pyxeleditor and install_pyxel_examples commands are not installed properly
I tried to run the Pyxel Editor with a fresh Pyxel install, and the commands seem to not be registered correctly.
![image](https://user-images.githubusercontent.com/16415678/47737809-0d296800-dc72-11e8-92dc-6d97c30fc789.png)
Same error for install_pyxel_examples.

 - OS: Ubuntu 18.10 Cosmic Cuttlefish
 - Python: 3.6.7rc1
 - Shell: fish