Generate strings

Generate NMEA strings based on a dictionary of properties. 
Properties: 
- time of fix (when was the fix acquired)
- location (latitude and longitude) 
- date
- whether the string is OK or not
- speed
- course
- [magnetic variation](http://en.wikipedia.org/wiki/Magnetic_declination)

It is also possible to use a generator for creating these strings. You can pass in a date and time that will alter time of fix and date, which allows some simple time travel. Other properties cannot be edited through the generator, use the object properties instead.