0 precision currency support in formatting

To format 0 decimal precision currencies, make a room to override
w2utils currency configuration.
Icelandic kr.
Japanese ¥
Korean ₩
Indonesian Rp
Malay RM
Uzbek so'm
Malay(Brunei Darussalam) $
Fix for null value handling in grid cell editing

null, undefined, empty and 0 values are properly handled in grid cell
editing.
Merge pull request #496 from JakeJP/master

0 precision currency support and null value handling