#26 Add copy of library for Julian Date conversions

Add the python module `jdutil` with many useful
date conversions, including conversion from
arbitrary dates/times to a numerical
floating-point representation that can be easily
stored in databases and other systems where we
need easy and reliably sorting and comparison of
arbitrary dates.

Note that I chose this module over other library
options available for Python because in my testing
it did the best job of keeping accuracy for
extreme dates (like year +-999,999,999) while also
generating using a number that is easier to store
than the two-float representation often used for
Julian dates.

Source: https://gist.github.com/jiffyclub/1294443
Credit to: Matt Davis, http://github.com/jiffyclub

changelog.rst

@@ -16,6 +16,10 @@ In development
       lower_fuzzy()
       upper_fuzzy()
 
+* Add `jdutil` library code by Matt Davis at
+  `https://gist.github.com/jiffyclub/1294443`_ to convert dates to numerical
+  float representations.
+
 3.0 (2018-02-13)
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