Updated DataChannelAuthentication methods and third party active modes. 
- Added methods to disable DCAU. 
- Active (3rd party) transfer mode between servers can now be reversed.
Added logger info statement.
Fixed active/passive mode bug: 
When performing passive() on source server, target server must use port
number from source server's passive() method and not use
setLocalPassive() method.
Cleanup: do not superfluous setLocalPassive() on target server!.
Changed usePassive property to useActive. 
Update comments + property descriptions.
Comments,exceptions,logging,etc.
Merge branch 'develop' of github.com:NLeSC/Xenon into feature/gftp-3rdparty-copy
Added check for null argument.
Formatting + comments.
Moved third party transfer methods to GftpThirdPartyTransfer class. 
Added support for extended mode partial third party transfers.
Changed dispose into close method.
Formatting.
Fixed weird line break.
typos
Fixed '&&' vs '&' in boolean statement. 
+ Comments and formatting.
Merge pull request #205 from NLeSC/feature/gftp-3rdparty-copy

Finished GftpSession API for 3rd party file transfer