Allow overriding default classes on th and td elements

This release adds a cellClass option to add class to the td elements, and modifies the headerClass setting to override the default of adding the field key as a class as well. If you're using headerClass and you want the field key as a class, you'll need to add it to headerClass explicitly.

It addresses #161 - using keys that matched bootstrap classes could add unwanted styling to the table. Set headerClass and cellClass to empty strings to prevent this.