Remove ghost.js

fixes #1575
- Moves most code that was in ghost.js into ./core/server/index.js
- Creates ./core/server/config/theme.js to hold all theme configurations 
(which previously lived on ghost.blogGlobals())
- Removed ghost.server, passing it in as an argument where needed 
and allowing middleware to hold onto a reference for lazy use.
Merge pull request #1630 from hswolff/remove-ghost-singleton

Remove ghost.js