Tap output, closes #31

* Do not fail the whole test suite if a single feature fails
* Replace text reporter with TAP one
* Write the TAP report to /tmp/zucchini.t

The report now looks like this:

```
1..2
not ok 1 - add_cancel
    1..2
    not ok 1 - 01_books.png does not match (no reference or pending screenshot for retina_ios6)
    not ok 2 - 02_new-book.png does not match (no reference or pending screenshot for retina_ios6)
    Bail out! Instruments run error
ok 2 - add_new_book
    1..6
    ok 1 - 01_books.png
    ok 2 - 02_new-book.png
    ok 3 - 03_book-title.png # pending
    ok 4 - 04_author.png
    ok 5 - 05_new-book-filled.png
    ok 6 - 06_books-with-a-new.png
```