use helmet module to more secure application

Helmet module is the middleware to add security headers for Application.
I think these headers is so important for security that we should also use in MEAN.js.

We need to install the latest one from github currently because it looks old on the npm site.
The old one is not working on my local MEAN.js stack.
keep enable Cache-Control

helmet.defaults() disabeles Cache-Control. I guess it's controversial to disable it.
Add envvars for deploying to cloud providers
Add joining of multiple accounts
Add account buttons to join more accounts
Migration to Express 4.x

Please see the Express documentation for details.

connect-mongo NPM package is currently broken with Express 4.x. Use
this fix in the meantime:
https://github.com/kcbanner/connect-mongo/issues/103#issuecomment-400800
50 .
Merge branch 'pr/18' into 0.2.3
Merge branch 'pr/14' into 0.2.3
Merge branch 'pr/15' into 0.2.3
Fixed locals.cache when NODE_ENV==='production'
Move license to it's own file.  Easier to see what the license is when first encountering the repo
Update README.md to include where to download the MEAN.JS boilerplate
Merge branch 'pr/23' into 0.2.3
Fixing License and Readme
Fixed Comments and README
Merge branch 'pr/15' into 0.2.3
Finalizing the User Accounts Module
Adding Comments
Adding Accounts View
Merge branch '0.2.3'