Major code refactor.

Split two bulk jQuery loops into several more maintainable private
functions.
Added a new option: onlyColumns

Use `onlyColumns` to set which columns are included and ignore all
others. This takes presidence over `ignoreColumns` when provided.
Updated readme
Updated readme
Merge pull request #3 from lightswitch05/feature-only-columns

Added a new option to specify the columns that should be included and ignore all others