guess_column_types could guess better

Numeric fields are apparently assumed to not be integers, it seems,
because if you allow continuous integer columns, assumptions about
distinctness ratio are invalid.

Cardinality alone seems to be sufficiently useful as a metric
for guessing continuity in this context. Highly distinct
floats will still have high cardinality for any reasonably sized
dataset, and now continuous but bounded integer columns (think
of generally approximated measurements like age/height/weight/etc)
will not be guessed as categorical just for not having many
decimal places.
Merge pull request #56 from fiendish/100_categories_is_probably_wrong

guess_column_types could guess better