RFP change user

Added CRUD functionality for User Class
Your validations should be above your methods: https://github.com/bbatsov/ruby-style-guide#consistent-classes

The formatting of your validation could also use some work:

```ruby
validates :username, uniqueness: { case_sensitive: false }
```

or 

```ruby
validates :username,
  uniqueness: { case_sensitive: false }
```