Fix broken type inference from currency to float in Excelx

The type inference that Excelx#to_type does is broken for at least
one valid currency format. Check out this xlsx file:

http://scpike-drop.s3.amazonaws.com/broken.xlsx

 > Roo::Excelx.new("broken.xlsx")
 => {[1, 1]=>"1899-12-30"}

The problem is that EXCEPTIONAL_FORMATS doesn't handle every type of
currency format that excel does. This format code has a "d" in it
(from "Red") so it gets marked as a date. Rather than hardcoding the
list of that are floats, can we just assume all formats with # are
floats? My interpretation of §18.8.31 in the
[spec](http://www.ecma-international.org/publications/standards/Ecma-376.htm) says yes.

This is not a problem in Roo::Excel, just Roo::Excelx