Optimized eta_letters().

Updates tests since I no longer omit 0-value numbers in the middle of
the strings (e.g. 1day 0 minute 1 second).

From:
$ python -m timeit 'from eta_conversions import eta_letters as eta;
[eta(i, shortest=shortest, leading_zero=leading_zero) for i in (0, 5,
65, 3605, 86405, 604805, 2630000) for leading_zero in (True, False) for
shortest in (True, False)]'
10000 loops, best of 3: 165 usec per loop

To:
$ python -m timeit 'from eta_conversions import eta_letters as eta;
[eta(i, shortest=shortest, leading_zero=leading_zero) for i in (0, 5,
65, 3605, 86405, 604805, 2630000) for leading_zero in (True, False) for
shortest in (True, False)]'
10000 loops, best of 3: 50.1 usec per loop