Optimized for Windows OS

[moyamejiasr]

The actual version breaks all the epub generated files on windows.
This little fix will repair most of the problems on it.

[coveralls]

Coverage increased (+0.08%) to 81.354% when pulling 741728a on Onelio:master into b0e9199 on bmaupin:master.

[bmaupin]

It looks like filepath.ToSlash might be a cleaner way of handling this:

Use the function filepath.ToSlash to replace the operating system path separator with '/' in a path.

On Windows, the function returns strings.Replace(path, string(filepath.Separator), "/", -1). On other operating systems, the function returns the path argument as is.

(https://stackoverflow.com/a/40002602/399105)

What do you think?

[moyamejiasr]

Seems clearer for me also, use this one if you like.

[bmaupin]

Okay, I'll reopen your pull request since I think I can just modify it.

[moyamejiasr]

Looking it more deeply seems like the function does the same thing as my commit
https://golang.org/src/path/filepath/path.go#L165

[bmaupin]

I made the changes and pushed them to the windows-fixes branch. Can you make sure it works for you and then I'll merge? I don't have access to a Windows machine.

Thanks!

[moyamejiasr]

It works nice

[bmaupin]

I've pushed the changes. Thanks for your contribution!