Refactor frontend

* restructured JavaScript using backbone.js
* replaced highlight.js with CodeMirror for its editor
* added CodeMirror Solarized (dark) theme based on Ethan Schoonover's solarized.vim
* changed `POST /document` to accept real JSON
* cleaned up template and stylesheet

lib/document_handler.js

@@ -55,7 +55,7 @@ DocumentHandler.prototype.handlePost = function(request, response) {
     if (!buffer) {
       response.writeHead(200, { 'content-type': 'application/json' });
     }
-    buffer += data.toString();
+    buffer += JSON.parse(data.toString()).data;
     if (_this.maxLength && buffer.length > _this.maxLength) {
       cancelled = true;
       winston.warn('document >maxLength', { maxLength: _this.maxLength });