[WIP] Assets

We need a mechanism to deliver static assets from themes/plugins which won't be stored in web server document root. Asset handler will generate static asset from given file or glob under `/static` url. Later there will be option to use specific domain to optimize server load.

Based on kriswallsmith/assetic lib.

usage:

```<link rel="stylesheet" href="{{ asset glob="_css/*.css" }}" />```
merges all css files under `_css` folder and create single file

```<link rel="stylesheet" href="{{ asset file="_js/init.js" }}" />```
generates single file based on path and mtime

```<link rel="stylesheet" href="{{ asset file="_img/logo.png" target="logo.png" }}" />```
generates file with given filename e.g. `/static/logo.png`

todo:
* [x] Handle file/glob assets,
* [ ] Support for css filters - less/sass,
* [ ] Support for image filters,
* [ ] Support for js filters - min/closure,
* [ ] Add type specific template tags - `js/css/less/sass`,
* [ ] Support for `STATIC_URL` setting.