[99] pydot should quote labels with colons

[prmtl]

Reported by nitsuadi...@gmail.com, 2015-01-31T11:26:23Z

What steps will reproduce the problem?

1. add to a graph pydot.Edge(src='A', dst='B', label='A:B')
2. try to render

What is the expected output? What do you see instead?

Expect a graph. Instead, produces an invalid DOT file which then leads to an error:
Warning: /tmp/tmpxw5_0w: syntax error in line 5 near ':'

What version of the product are you using? On what operating system?

1.0.28 Linux (Ubuntu)
From: https://code.google.com/p/pydot/issues/detail?id=99