Added labels to outgoing control flow edges of condition nodes.

The additional information is stored in a separate edge class, which can
store arbitrary data in form of key-value pairs.

The set of control flow graph edges is not accessible directly anymore,
but only through methods provided by the control flow graph itself.
Adapt unit tests to the latest changes.

To simplify this, a new method is introduced to check whether two
nodes are connected in the control flow graph or not.
Merge pull request #12 from a0x77n/dev

Control flow graph edge labels.