Add some css classes to the perseus API

Test Plan:
Added the following css to perseus.less:

    .framework-perseus .perseus-input {
        background-color: blue;
        &.perseus-focused {
            background-color: green;
        }
    }

And checked that the input-numbers and expressions showed up
blue when unfocused and green when focused

Reviewers: laura, charlie

Reviewed By: charlie

Subscribers: alex, mgp

Differential Revision: http://phabricator.khanacademy.org/D9503