Allow periods in secondary index column names

Premise: it is natural to map JSON objects to column names using periods to denote the object hierarchy.

For example {food:{fruit;

}}
Would map to the following CF names: food.fruit.a, food.fruit.b, food.fruit.c

However, secondary index names cannot contain periods which means workarounds (ie denormalization) must be used to index such columns.

It would be nice if this restriction could be removed.