nitdoc: escape text in generated dot files

Unescaping names in dot files generated by nitdoc seems causes the recent failures on jenkins.

The bug was present since the new nitdoc but expressed by #883 that exposes the name of the project in the dot-file:
* On Jenkins, multiple workspace are identified with a @. eq. `workspace@2`
* The name of a Nit project is the name of the directory
* Therefore  the `dot` command aborted when an identifier with @ was found, thus did not produce a map file.
* Therefore nitdoc aborted on the opening of the map file (to include in the HTML)

Retated to #891