Add support for dynamic partial names

Uses the subexpression syntax to allow for dynamic partial lookups. Ex:

```
{{> (helper) }}
```

Fixes #933

lib/handlebars/compiler/javascript-compiler.js

@@ -644,17 +644,26 @@ JavaScriptCompiler.prototype = {
   //
   // This operation pops off a context, invokes a partial with that context,
   // and pushes the result of the invocation back.
-  invokePartial: function(name, indent) {
+  invokePartial: function(isDynamic, name, indent) {
     var params = [],
         options = this.setupParams(name, 1, params, false);
 
+    if (isDynamic) {
+      name = this.popStack();
+      delete options.name;
+    }
+
     if (indent) {
       options.indent = JSON.stringify(indent);
     }
     options.helpers = 'helpers';
     options.partials = 'partials';
 
-    params.unshift(this.nameLookup('partials', name, 'partial'));
+    if (!isDynamic) {
+      params.unshift(this.nameLookup('partials', name, 'partial'));
+    } else {
+      params.unshift(name);
+    }
 
     if (this.options.compat) {
       options.depths = 'depths';