Use named subject
Avoid `is_expected` in multi-line `it` block
Use `let` instead of instance variable
Reorganize RSpec hooks
Make username of user factory to sequence
Validate presence and uniqueness of username
Merge pull request #83 from yous/patch-rspec

Make RSpec code more self-descriptive
Merge pull request #84 from yous/patch-model-rspec

Add validations to User