Don't allow nodes to overlap edges or other nodes

We allow Graphviz to reposition nodes slightly and draw curved edges to
make this possible.

Fixes #3. Also fixes some overlap seen in the
howto/separating-topic-branches.txt diagrams.

lib/asciidag.rb

@@ -50,6 +50,7 @@ def initialize(nodes, branch_labels)
     def dot
       result = []
       result << 'digraph {'
+      result << '  graph [splines=true, overlap=false];'
       result << '  node [shape=circle];'
       nodes.each do |node|
         x, y = node.position