Implementation of `JulianDate.toIso8601` and `JulianDate.toGregorianDate`.

1. `JulianDate.toIso8601` creates an ISO8601 string with the specified precision.
2. `JulianDate.toGregorianDate` returns a data structure with individual date components which can be used for custom formating.
3. Added `sprintf` function via third-party code.  The code is MIT and their license/copyright notice is here: http://phpjs.org/about/  I wasn't sure where I'm supposed to add it, in the code itself or in License.MD?  @shunter?

This addresses #399 

CC @emackey and @chris-cooper