Bug 28985: Force positive numbers for itemtype charge fields

Bug 28985: Force positive numbers for itemtype charge fields

This patch adds a min attribute to the fields on this page and adds
other cost related fields to validator

To test:
1 - Apply patch
2 - Browse to Administration->Item types
3 - Edit or create an item type
4 - Attempt to place a negative or non numeric value in:
    Daily rental charge
    Hourly rental charge
    Default replacement cost
    Processing fee
5 - You should not be able to

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>