[BUGFIX] Fixed implementation of preProcess() hook in 6.0

This update fixes the following issue:
https://github.com/NamelessCoder/flux/issues/19

See code comment in ext_localconf.php for more.
Merge pull request #21 from murderfacemurderfacemurderface/Issue_19

[BUGFIX] Fixed implementation of preProcess() hook in 6.0
[FEATURE] Add ViewHelper to convert raw FlexForm XML into an array
[TASK] Make uid argument optional
[TASK] Make arguments required, fix docblock
[TASK] Add simple validation for arguments
Merge pull request #22 from dreipunktnull/viewhelper/flexform/data

[FEATURE] Add ViewHelper to convert raw FlexForm XML into an array
[TER] 5.1.1