Added nails.json

This'll be a centralised config file for handling some stuffs. Will also use for auto-detecting core modules.

Also updated System_startup to utilise nails.json for version number.
Changed blog & Shop skin detection logic

Official skins are packaged up as composer packages and should be required by the app as needed.

The skin models will search various directories to find skins and optionally filter out sub directories using a regex.

As a result the getting started skins are no longer required in the core of Nails.
Moved all Nails modules out of common and into their own respective packages.
Updated composer.json to require as few dependencies as possible.

i.e just Nails admin and system modules
Bumped version number and updated CHANGELOG.md
Merge branch 'release/0.2.2' into develop