add virtual attr 'fullName' to user model

This commit adds a virtual attribute 'fullName' to the user model.
This combines the actual model properties of 'firstName' and 'lastName'
to provide a 'virtual' attribute of 'fullName'.
remove hard-coded db connection credentials

The mongolab db user creditials were hard-coded in the developmeent.js
config file, which was accessible to the public on github.  Even though
this is code only follows along with the book, I thought it would be a
good idea to refactor the credentials to environment variables and
source those to the operating system.

The db user creditials have been moved to a separate file which has been
added to .gitignore
Optimize queries using indexes

Adds a 'unique' field to the username property and indicates that the
email field can be used as a secondary index.
Define custom methods in user model