Make `tools.md` better suited for `pandoc -t man`

Like discussed in the mailing list, this is going to be `piqi(1)` man page. To see how the manual page will look like, run these commands:

```
$ pandoc -s tools.md -t man -o piqi.1
$ man ./piqi.1
```

I could not find any scripts to generate HTML documentation which is found in http://piqi.org/doc/, so please verify that these changes look well in the website too.