Fix min/max integer param validation when either is 0.

Bug introduced in commit 5b4c4cf9bc9619d51d7f2471d64896dcdd93707c .
InRangeValidator would not be appended because self.min and self.max are no
more strings, but integers.

Reported by: Andrea Pinna <andrea.pinna@crs4.it>
Use min/max <param> attributes to validate integers/floats also when only one limit is specified.

<validator type="in_range" /> already works with only one limit specified.
Also check that min <= max .
Merged in nsoranzo/galaxy-central (pull request #254)

Fixes for min/max <param> attributes