Add DateTime to_rfc3339p(z) methods, tests
Add DateTime to_rfc3339p(z) methods, tests

These additions allow convenient control of RFC 3339 formatted output:

 * Number of subsecond digits to display

 * Whether to use the 'Z' variant, instead of "+00:00" for TZ offset
   0, UTC.

...while remaining faithful to the RFC 3339. The implementation uses
the existing formatting Item mechanism.

github: cc: #157 #178

lifthrasiir/rust-chrono