Support for single char 24h HH vs H

I added support for a single H format which allows to have single character 24 hours formatting. Similar to the Java version (see below).

http://docs.oracle.com/javase/1.4.2/docs/api/java/text/SimpleDateFormat.html

H	Hour in day (0-23)	Number	0


Let me know if you want me to do anything else in regards to doc's or tests.

Best, Mirko