Dash (-) in directory names is not escaped/quoted in DOT identifiers

[torfsen]

pyan constructs the DOT identifiers using the filenames. If a directory name contains a dash (-) then that ends up in the DOT identifier. However, the DOT language does not allow that, unless the identifier is quoted. Hence the produced DOT file is invalid.

[akashkvyas]

Submitted a pull request that fixes this issue.