Log the full stack trace when a test fails

If a test fails inside a function other than the test function, print the full stack trace leading up to the failure. Example:

```
1..1
not ok 1 failing helper function
# (from function `failing_helper' in file /Users/sam/Projects/bats/test/fixtures/bats/test_helper.bash, line 6,
#  in test file /Users/sam/Projects/bats/test/fixtures/bats/failing_helper.bats, line 5)
```

Fixes #14. /cc @mislav