vw -b 32 does not work

This should solve issue #450.
Not only --invert_hash, but also -f produces an empty model (even on machines with 256GB RAM).
I am not aware of application needing 32 bits hash (once -b 29 gave me the best results).
However, some users try to use -b 32, so we should produce a clear error message.
Merge pull request #456 from martinpopel/issue450

vw -b 32 does not work