Military (24-hour, ISO8601) time support

Hi there,

I implemented military time support

* new formatting parameters: `H` and `HH` for military time hours
* period formatters used in conjunction with military time formatters are ignored
* new getter/setter `militaryHours()` and `militaryHours(hours)`
* added a new method `toISOString()` that returns the ISO8601 time component (without seconds)
* added military time tests
* removed a couple existing tests that were military time and now work correctly

It would be great if you would review and pull this in.

Thanks,
Kimon