FIX: NumericField should work with numbers like 54,6

Fixes http://open.silverstripe.org/ticket/5577.

Uses Zend_Locale_Format::isNumber(). Includes unit test for NumericField. Does not include testing work on DBField underlying NumericField to ensure that works consistently.
Merge pull request #1877 from wilr/5577

FIX: NumericField should work with numbers like 54,6