Fix microseconds formatting

[fsabe]

This fix formatting fractional seconds with leading zeroes (issue #388) and change the format of microseconds when using %f to conform with what's documented:

%f - microseconds in zero padded decimal form, i.e. 025000

Please see the changed tests. I'd like to hear some thoughts on whether the precision should be taken into account when formatting microseconds.

Summary of changes

• Pad with zeroes before truncating when formating fractional seconds;
• Pad with zeroes and never truncate when formatting with %f.

[coveralls]

Coverage increased (+0.1%) to 68.098% when pulling 7f27d30 on fsabe:format-us into c86a96a on bitwalker:master.

[bitwalker]

Thanks for the fix!