Add user model

This pull request depends on a previous pull request: Devise Initialisation (https://github.com/abhishek-reddy/GemExpo/commit/e776c73c1e1da30cf5ff60d4385bbb59977ca8c1). Please consider reviewing and merging Devise Initialisation Pull Request before the current Pull Request.

This Pull request adds a user model to the existing Devise Initialisation Pull Request. The Sign in page which is provided by default in devise gem is added to the view.