coding style fixes, some PHPStorm inspection fixes, improved readability of code, few refactorings, all as part of our code cleanup strategy

Piwik is a free/libre analytics platform. Refs #4455 GNU Package requirement to use free/libre instead of open source

using phpstorm optimize imports feature

using phpstorm optimize imports feature

moving getMaxCustomVariables to CustomVariables class, a model should not really care about caching etc

Fixes #3116 If website is ecommerce, do not limit custom variables reports to 1,000 rows. Limit to 50,000 instead.

fixes #4609

Added namespaces to functions.php files in the five plugins that had one
Work in progress

renaming Unzip to Uncompress,
Renaming Tracker/Referer to Tracker/Referrer,
and few more

Removed many Piwik_ functions, in Piwik 2 it is best practise to use the methods calls instead

Todo: finish converting core/ classes + convert plugins/ classes to use \Piwik\Plugin namespace + fix build + Merge master

Refs #4059 Work in progress: Conversion to use Namespaces: Period*, Metrics, Segment, SegmentExpression, PluginsManager.
Removed some deprecated code.

Notice that auto refactoring has created a nested namespace. Not sure this is what we want - so we might have to edit those nested namespaces afterwards (I think they don't look so good)

Notice that auto refactoring has created a nested namespace. Not sure this is what we want - so we might have to edit those nested namespaces afterwards (I think they don't look so good)

2 steps required:

1. Refactor > Move > \Piwik\Core\Config (uncheck "Create new File")
2. Refactor > Rename > Piwik_Config to Config

Also moved some other const to Piwik class.

such as removing duplicate code and a lot of refactoring, the code is now much more readable!

Removing what seems to be unused code? @sgiehl you added it but I cant find any code calling generateDatatable.