AddAll with includeCurrentFolder=false has leading slashes on subdirectories in the archive

[gavbaa]

Given a simple zip creation setup like:

    zipFile := new(archivex.ZipFile)
    buf := new(bytes.Buffer)
    zipFile.Writer = zip.NewWriter(buf)
    zipFile.AddAll("/tmp/dir", false)
    zipFile.Close()

The AddAll specifies includeCurrentFolder=false. There's no issues when it's true.

The produced .zip gives me the following when I attempt to unzip it.

➜  unzip -t blah.zip
Archive:  blah.zip
    testing: a.xsd       OK
    testing: b.xsd              OK
    testing: /thing/                   OK
    testing: /thing/blank.htm          OK
    testing: /thing/easyxdm/           OK
    testing: /thing/easyxdm/MIT-license.txt   OK
    testing: /thing/index.htm          OK
    testing: /thing/js/                OK
    testing: /thing/js/API.js   OK
    testing: /thing/js/Utils.js        OK

The leading slashes should not be there, and could cause issues with some zip clients.

I've submitted PR #17 to address it.