Update 404 page.

Center all the things.
Fix code highlighting in Securing Passwords post.

For code highlighting to work with PHP, there needs to be opening tags.
Better gulp serve task

Use browser-sync instead of Jekyll --watch. Found that in some cases,
Jekyll would continue to run even after gulp had been exited.

Don't use gulp build to build the site anymore. This does not run any
requirements. I think this was the cause to them running twice before
using serve. Use the default task to build the site.

Renamed javascript task to script.

Added some comments to tasks to explain what they do if the name wasn't
enough.