Add a `DotGraph` and `DotNode` classes, which implements `GraphTraits`,
so that we can create a `RegionCFG` starting from a graph specified in
a `.dot` file.
The `DotClass` implements a minimal parser for graph specified in `.dot`
format.
The `.dot` should begin with the specification of the name of the graph
`digraph TestGraph {`, followed by an arbitrary number of lines which
specify the edges in the graph (no attributes allowed, e.g., `a -> b;`).
The file should end with a single line ending in `}`.
Andrea Gussoni