Add UI support for worksheet directives

Node, dependent on: https://github.com/codalab/codalab-cli/pull/21

This change adds support for worksheet directives in the UI (see: https://github.com/codalab/codalab/wiki/Dev_Displaying-Bundle-Contents-and-Metadata).

For instance, the following worksheet markdown translates to the following worksheet page:

```
# Sample
This is a sample containing the following content: Markdown, image, table with column-headers, table with row-headers and a hidden bundle to provide data for images and tables.

// image: image/image.png

// display table
// Bundle Creation: metadata/created
// Bundle Size: metadata/data_size

// display inline
// Bundle Creation: metadata/created
// Bundle Size: metadata/data_size

[]{0xef98ccb37e0d42789ef3aa3fa4df1b85}
```

![worksheetdirectives](https://cloud.githubusercontent.com/assets/3478847/2784279/cb549f88-cb39-11e3-9dd0-d857eeac05aa.png)