Reduce heap exhaustion during export (and partly import)

This patch mainly provide the following improvements:

1) Stop building DOM in memory for just streaming them
2) Avoid buffers duplications in favor of stream filters

These are involved in

1) Packaging
2) ZipExplorer
3) Upload
4) Attachment contents

This patch require an upgrade of org.apache.commons.codec to version 1.4 (dependency against Base64OutputStream)

see http://www.mail-archive.com/devs@xwiki.org/msg10988.html for more details