More implementation.

Modified intent of ts_db_records. timestamp value is no longer a
  {Date, Time} value, it is now gregorian seconds since year 0.
  This was done to make queries that are based on date comparisons
  easier to write and more efficient.
Added date time comparison function to ts_common.
Implemented update/1 and list/3 for ts_entry.

src/ts_db_records.hrl

@@ -6,7 +6,7 @@
 
 -record(ts_entry, {
     ref,        % {username, timelineid, eventid}
-    timestamp,  % {{year, month, day}, {hour, minute, second}}
+    timestamp,  % gregorian seconds
     mark,       % String description of entry
     notes       % String with further notes about the entry
 }).