CLEANUP (tools): remove Gruntfile.js

- Remove Gruntfile.js and dependencies as we now use Gulp
TOOLS (gulp): add gulp build function

- add gulp build function, uses `node-webkit-builder`

NOTE: need to run `npm install` to install new dependencies
FEATURE (editor): add preview of markdown

- add preview of markdown files (files with .md extension)
STYLE (css): update text fields to reflect invalid state

- Update text fields to reflect invalid state. While invalid - border is red. Works on :focus only.