Added animal feature spec

The only file you need to look at here is [spec/features/animals/new_animal_spec.rb](https://github.com/s4g3/zookeeper/blob/93573b926c279f1905ae0468dd031d8df6cbbd73/spec/features/animals/new_animal_spec.rb). Once you've merged this branch and sync'd with your local repo, you should run `bundle` followed by `rake`. You should see one test fail and one pass.

```
$ bundle
Using rake (10.1.1)
Using i18n (0.6.9)
[...]
Your bundle is complete!
Use `bundle show [gemname]` to see where a bundled gem is installed.

$ rake
ruby -S rspec ./spec/features/animals/new_animal_spec.rb
.F

Failures:

  1) Create a new animal A user submits a invalid animal
     Failure/Error: expect(page).to have_content "Name can't be blank"
       expected to find text "Name can't be blank" in "Animal was successfully created. Name: Species: Weight: Notes: Edit | Back"
     # ./spec/features/animals/new_animal_spec.rb:23:in `block (2 levels) in <top (required)>'

Finished in 0.15479 seconds
2 examples, 1 failure

Failed examples:

rspec ./spec/features/animals/new_animal_spec.rb:19 # Create a new animal A user submits a invalid animal

Randomized with seed 56980

ruby -S rspec ./spec/features/animals/new_animal_spec.rb failed
```

To see more details of the tests run `rspec -fd`.

Let me know when you've got the test suite running (and failing as it should) and I'll raise an issue for making the currently failing test pass.