save modification time with sub-second granularity

Some systems can get the file modification time with sub-second
granularity. However, Hakyll shaves off the sub-seconds, as defined in
the Binary instance of BinaryTime, which poses a problem because when a
file is checked to see if it was modified in `resourceModified`, it
still contains the sub-seconds. This results in a file (almost) always
being considered as having been modified.

Example:

1. First go around, modification time is 3:45.325. This time is saved
   as 3:45.000 (i.e. sub-seconds are shaved off).
2. Second go around, modification time is again read as 3:45.325 and
   compared against the stored time, 3:45.000. 3:45.325 is more recent
   than 3:45.000, so the file is considered to have been modified.

This change prevents the shaving off of sub-seconds. This will naturally
work on systems that don't support sub-second granularity, as that
'field' will simply appear as all zeros.

Closes #250
Merge pull request #252 from blaenk/mod-time

save modification time with sub-second granularity