Added new ant target docs: "Generate documentation"

```
New ant target is added to build.xml
name: "docs"
description: "Generate documentation"
```

Original issue reported on code.google.com by `natry.cl...@gmail.com` on 20 Sep 2013 at 10:32

Attachments:
- [build.xml.patch](https://storage.googleapis.com/google-code-attachments/s2-geometry-library-java/issue-2/comment-0/build.xml.patch)
