Rename config-system.el to configuration-layer.el
Random selection of startup banner
Add a templare for layer README
Update CONTRIBUTE.md for banner submission and layer readme template
Add variable dotspacemacs-startup-banner

To choose between:
- random for randomly chosen banner
- an index for a specific banner
- nil for no banner at all
Remove Ensime from company-mode-excluded-packages

Resolves #305
Better detection of battery absence
Display chosen banner in Messages
New function `configuration-layer/layer-declaredp`
Move window-numbering package config into its own init function
Neotree window number is now always 0

This other windows keep their number