Dependency graph vizualization

I have enhanced the repo-dep-tree.pl script with producing a [DOT](http://www.graphviz.org/content/dot-language) file that can be coverted into ![PNG image](https://github.com/yurloc/droolsjbpm-build-bootstrap/blob/dep_graph_img/src/main/resources/dep-tree.dot.png) using Graphviz.

Also, the script will print a list of repositories required to be built before a given target repository (in correct order):
```
You need to build following repositories before building 'kie-wb-common' (in order):
droolsjbpm-knowledge,drools,jbpm,droolsjbpm-integration,kie-uberfire-extensions,guvnor,kie-wb-common
```
The target repository may be specified using `-t` option.