fix sorting on numeric columns

Addressing two issues with sorting columns containing numeric values: 

1. javascript error when trying to sort -- TypeError: Object 3.45 has no method 'toLowerCase'
2. separate out blanks from 0's

Edit: it looks like there's a separate pull request for the first issue (https://github.com/javve/list.js/pull/160).  I'll leave the commit list unchanged, just in case there are any incompatibilities.