.list-group and .table appearance fixed when inside .panel-collapse

I found out `.list-group` and `.table` didn't behave the same when put inside a `.panel > .panel-collapse`. Have a look and consider my pull request.

Also I removed the `border-top-width` definition for `.panel > .list-group` and `.panel > .panel-collapse > .list-group`, so they appear exactly like `.table`s.