Emoticon support

This adds support for "emoticons" per #3.

A script replaces (for example) `:-)` in message text with `<span class="subrosa-image-emoticon" title=":-)"></span>`.

Currently there are a couple of basic emoticons recognized. To add more, put images in [./img/emoticons](https://github.com/rschaosid/subrosa-client/blob/master/img/emoticons), add text aliases in [./js/app-emoticons.js](https://github.com/rschaosid/subrosa-client/blob/master/js/app-emoticons.js) and add styles in [./css/image-emoticons.css](https://github.com/rschaosid/subrosa-client/blob/master/css/image-emoticons.css).