Add glob method to PathHelper which escapes \ characters

This should solve the tummy ache caused by Windows paths which use the `"\\"` separator.

See: https://github.com/opscode/chef/issues/2078

\cc @opscode/client-engineers 
Can you add a test that calls `Dir.glob` to verify it works on Windows? When I try this code out on Windows, it's not working. I put the pattern substitution into the function `safe`, but it seems like any input to `Dir.glob` with a backslash in it fails.

```
irb(main):016:0> Dir.glob("lib/*.rb")
=> ["lib/chef.rb"]
irb(main):017:0> Dir.glob('lib\*.rb')
=> []
irb(main):018:0> Dir.glob('lib\\*.rb')
=> []
irb(main):019:0> Dir.glob(safe("lib\\*.rb"))
=> []
irb(main):020:0> safe("lib\\*.rb")
=> "lib\\\\*.rb"
```

Let me know if I'm missing something or otherwise Doing It Wrong.