Adding quotes to DOT keywords

In DOT language, the keywords node, edge, graph, digraph,
subgraph, and strict are case-independent.  When adding
a node with a DOR keyword as its name, the node's name must
be surrounded with double quotation marks, otherwise it
produces an invalid DOT file.
Signed-off-by: Paulo Roberto Urio <jimmyskull@gmail.com>
import 0.3.4 release