Various optimizations

- Added `docpad-plugin-winston`
- Switched from `docpad-plugin-sass` to `docpad-plugin-nodesass`
- Added `preinstall` script to clear npm cache
- Added standard grunt commands:
  - `grunt production`
  - `grunt run`
  - `grunt deploy`
- Added `npm-shrinkwrap.json`
- Removed unnecessary dependencies:
  - docpad-plugin-marked
  - docpad-plugin-partials
  - docpad-plugin-stylus
  - docpad-plugin-text