Only style as striped table when .backgrid-striped class is applied

The `.backgrid` class applies table striping styling which is incompatible with [bootstrap table styling](http://twitter.github.io/bootstrap/base-css.html#tables). Would it make sense to pull the striping style out into a separate css class `.backgrid-striped` as is done in this PR?

E.g. in my case, I wanted to apply the bootstrap row class `.info` which would change the background color; however this does not change the background color of odd rows due to the backgrid css. With the change in this PR however, my application can remove the default `.backgrid-striped` class and things seem to work as hoped (I believe using a space-separated string of class names for backbone `className` works as expected.)