Updated 24hour detection

Updated 24hour detection to look for lower case 'h' (12-hour clock hours) as well as case-insensitive 'a' (AM/PM marker)
Fixed comparison

We're looking for cases where 'a', 'A' and 'h' are not in the format string. They can be in first position so need to check for 'not found', i.e. `<0` or `-1`.
Merge pull request #483 from caponica/24h_detection_by_hour_format

Updated 24hour detection