Fixed crash when operator specified on integer field.  Added support for operators such as "<", ">=".  Added replace_row callback.

Operator filters are now supported for integer columns.  You can specify "< 3.2",">= 12", "=11", or just plain "11".

Fixed issue where rails threw an exception if a filter was specified such as "not a 3 number" or with an operator "< 12"

Added replace_row which is a callback like before_row or after_row.  It lets you specify the whole contents of a row, just in case you need to do something special like make a column that spans the whole row.