Merge pull request #3 from Carl4/master

More Unit Tests and better consistency in new object models
add pytz as an requirement
fix tests for python 2.6
iteritems is not available in python 3 anymore
fixes for python 3.2
Altered the behavior of "Number" when no time information is provided and the number is a scalar.
Added "Reference Frame" to the list of parameters for Position.
Merge branch 'master' of https://github.com/cleder/czml
Fixed a bug in the Number class.