Merge tag '3.8.2' into develop

3.8.2
Preparation for new development cycle after release 3.8.2
Fix warning Declaration of x should be compatible with Walker
Use of undefined constant directory - assumed 'directory'

closes #1
Update version
Undefined variables

Reformat code in the process
Multiple Undefined indexes
No need for passing object by reference.

We only support PHP5, so we can get rid of the passing by reference.
Reformat the code
Bugfix: Widget would not keep checked categories

Closes #3
Refactor code to PSR-2 standard

We try to code as close as we can to PSR-2, because WordPress doesn't use PSR-2 the code can;t be completely PSR-2 complaint.
Update changelog
Explain contributing to the plugin
Add info about contributing
Version bump 3.8.3-rc.1
Version bump 3.8.3
Merge branch 'release/3.8.3'