Append factor levels in within.ffdf()

This ensures that levels which do not appear in the first chunk
are not discarded.
Fix ffdfappend() when appending an integer to a factor

The code checked whether a factor was appended to a non-factor,
but not the other way round. In that case, some levels got lost,
and warnings were printed:
ram2ffcode(value, fflev, vmode): unknown factor values mapped to NA
Merge pull request #22 from nalimilan/master

Append factor levels in within.ffdf()