fix path in expansionFile.getAssetFileDescriptor

First, thank you for this plugin :)

In the reader, the expansion filename is repeated
```javascript
expansionFile.getAssetFileDescriptor(fileName + "/" + filename);
```
This returns a path like _main.1.com.company.appname/image.jpg_. 
This is working if we have zipped a directory called _main.1.com.company.appname_.

But, in many cases, content is added directly to the zip, with no deps
on the expansion file name.
We should use only the filename
```javascript
expansionFile.getAssetFileDescriptor(filename);
```
to return only path
_image.jpg_
This is a cleaner way to get the content.
If your zip file contains a directory called _main.1.com.company.appname_
you should now use it like that
```javascript
XAPKReader.get('main.1.com.company.appname/image.jpg', successCallback, [errorCallback], [fileType]);
```