Convert string to number

[talentedandrew]

Hi,
First of all thanks for this amazing library.There was an issue I was facing when using string values with cast.sum method.The problem was that my string values were not converted to numbers before summing them. So I found a method to do that. I modified the cast.sum method.I added a + sign infront of row[k] on line no. 92 in melt.js file.So now it looked like return a + (+row[k]); .Now , even with string values it worked fine. Please suggest some other way if its not appropriate.