Add database_cleaner

database_cleaner will delete all entries in database before each example.
With this, we can avoid data conflict between examples.
Change default format to `documentation`

With this, rspec command will output full documentation of each
specs instead of dots. This is useful to know what's happening
in our specs.
Add first spec
Add .travis.yml
Compare times with `#to_i`

`Time` is compared by fractional.
http://railsware.com/blog/2014/04/01/time-comparison-in-ruby/