Fix content for folder entries.
Using the DEFLATE compression on folder entries with empty content cause the Java zip inflator to fail.
This fix totally remove content when compressing a folder entry, and set compression to none for this entry.
Fix indentation with 3 spaces
Update jszip.js

Check data nullity in asText() to avoid errors
Merge pull request #33 from feugy/master

Handling folder entries for Java programs