adding support of other systems of time and converting from each one to other

[serggzz@…]

For ex. I need GPS time system. Short description is available at ​http://leapsecond.com/java/gpsclock.htm In gps it is rather convenient to have converting function from gps time in form of weeks cycles days and seconds (see example at site above noted) to UTC time. More information can be obtained from various gps-related sites or you can ask me, I'll help according to my knowledge. Leap second is noted in the library documentation, but really there is not any support. I guess that it is better to make function to set current leap seconds value since it is not predictable.