Added getMode method to determine the chmod on unix systems

hey there, 

I needed to access the chmod value from the central directory file header in order to get a mac app to unpack so I added a getMode to the Entry and the mode to the Entry constructor.

My sample file was https://s3.amazonaws.com/node-webkit/v0.6.3/node-webkit-v0.6.3-osx-ia32.zip

Here is the sample code to unpack:

```javascript
var ZIP = require('zip');
var fs = require('fs-extra');
var data = fs.readFileSync('node-webkit-v0.6.3-osx-ia32.zip');
var reader = ZIP.Reader(data);
reader.forEach(function (entry) {
    var mode = entry.getMode(),
        fileName = entry.getName();
    if(entry.isDirectory()) {
        fs.mkdirs(fileName, function(err){
            if(mode) fs.chmodSync(fileName, mode);
        });
    } else {
        fs.writeFileSync(fileName, entry.getData());
        if(mode) fs.chmodSync(fileName, mode);
    }
});
```