[FEATURE] Add ExtractViewHelper

ViewHelper to extract values by key from a nested array structure.
[DOC] Doc comment for Page / SiteUrl ViewHelper
[DOC] Doc comment for Page / Menu ViewHelper
[FEATURE] Add media related base VHs, add image related VH, add video VH
[TASK] Move image and video VHs to ViewHelpers\Media subpackage
[TASK] Fix wrong placement of class properties
[BUGFIX] Remove src argument from base class
[TASK] Check if $files is array in clearCacheCommand() of AssetViewHelper
[DOC] Update copyright year, Iterator / Values ViewHelper
[TASK] Remove unused attributes "start" and "length" from Iterator / Values ViewHelper
[FEATURE] Form / HasValidator ViewHelper (Condition)
[TASK] Use Form / HasValidator ViewHelper as parent of Form / Required ViewHelper

The "Required" ViewHelper is simply a check for HasValidator using NotEmpty as validator name. Every checking capability is contained in HasValidator.
[TASK] CGL cleanup, Random / Number ViewHelper
[TASK] Enable respect for "as" attribute in Iterator / Shift ViewHelper
[TASK] CGL cleanup and slight refactor, Iterator / Merge ViewHelper
Merge pull request #41 from mostwanted1976/master

[TASK] Check if $files is array in clearCacheCommand() of AssetViewHelper
[TASK] Refactor menu base class in preparation for new VHs
[TASK] Fix indentation
[TASK] Fix usage of obsolete method
[FEATURE] Add list menu VH as discussed in #44