Add auth

This pull request depends on the previous pull request - Adding user model (https://github.com/abhishek-reddy/GemExpo/commit/d65a0ae5946d7813d57f610e51a8797ae4fef202), so i would request you to review and merge that pull request before merging this one.

This Pull Request adds the functionality of Sign-in, Sign-out and Edit User. The forgot password functionality is not yet working and will be hopefully added in the next pull request.