git and github layers merge, github-browse-file, fixes and tweaks

• git and github layers have been merged into one git layer. To enable Github support set to t the variable git-enable-github-support in your dotspacemacs/init function.
• New package github-browse-file in git layer. Press SPC g f b to browse the current file in Github (thanks to rphillips)
• New mnemonic key bindings for windows split. SPC w - to split horizontally and SPC w / to split vertically. The old key bindings are still available (thanks to trishume)
• Fix read-only scratch buffer (thanks to herbertjones)