Support multiple units in format_timedelta()

[hartmutobendorf]

Issue #728 describes returning measures for several units when using format_timedelta().

E.g. instead of

10840 seconds
3 hours

return for depth=="fullest"

3 hours 0 minutes 40 seconds

and for depth=="full"

3 hours 40 seconds

Tests are passing, but I haven't tried contributing to babel before, and am not aware of any additional requirements beyond the few lines of source code in this PR. Feedback appreciated.