Custom and dynamic row classes and optionally remove odd/even classes

For your consideration: another way of specifying classes for rows, see issues #21 and #24. In my solution we do this by calling `row_class` in the block passed to `table_for`.
You can use a string or pass a block to create dynamic classes. 

Syntax may be a bit suboptimal, since you shouldn't output the result of the method call. 

I also added an option to omit the odd/even classes by passing an option to `row_class`, as per issue #28