Add missing + operator to time_point_sec

time_point_sec had an operator defined for time_point_sec -
microseconds, but not one for time_point_sec + microseconds. Now it
does.
Tweak FC enum reflection

Previously when enum values not defined in the original enum type were
used in serialized objects, FC threw exceptions when it encountered
these values. Now it just serializes unknown values to and from numbers
instead of names.

As an added benefit, the compiler now warns you if a value is defined in
the enum, but not reflected!