Vectorized DateFromJulianDay and JulianDayFromDate

This should mitigate Issue #296 by vectorizing DateFromJulianDay and JulianDayFromDate.

For the vecorization of JulianDayFromDate I had to implement a case for netcdf.datetime and one for regular datetime. Since properties are used the calls to the lambda functions were very expensive when getting the information out of the netcdftime.datetime object.
I now use the _ attributes directly if a netcdf.datetime object is given. 

I also tried to change the netcdf.datetime class to not use properties but to enforce the immutability by overwriting the __ setattr __ method but this slowed down the num2date case when netcdf.datetime objects had to be created.

Issue #75 also caused some minor problems. I fixed it by adding half a second to the julian date when getting the day part and the fractional day. This works but I am not sure that we should actually go to the next day if the jd is not >= 0.5?

My development environment also runs autopep automatically so I hope you don't mind all the small added spaces around stuff.

I think the basic functionality should still come with the netCDF-python package. I'm thinking about if I should write something specifically for the use case of reading time series data which might use numpy datetime64. Similar to what you already did. 

Something that is missing as far as I know is a way to get from year, month, day etc. arrays to a datetime like object fast.