Initial implementation of calculation of astronomical Julian Date.

* Added 'to_julian_date()' to Timestamp
* Added 'to_julian_date()' to DatetimeIndex
* Added tests for both methods.

Toying with the idea of using pandas for my astronomy and tidal analysis packages and I need the astronomical Julian Date.  So here is an implementation and tests.  I think I put these in the right place.  Really wanted to have the same function for the Timestamp and DatetimeIndex so that both would benefit from improvements, but couldn't figure out how to make that work.

Kindest regards,
Tim