Provide better information regarding memory usage by each plugin

I have implemented this issue: https://github.com/piwik/piwik/issues/6708
Also I have discussed this with issue author and it looks like good implementation of his idea. @mattab what you think? Should be more or this is enough for this PR? 
Did you maybe miss that we have `Formatter::getPrettySizeFromBytes` ?
before implementing something always search in the codebase. for example I search for `1024` and find all code that does stuff with bytes/kb and I found this function.