Refactor ProjectStatistics API

[jerguslejko]

This PR cleans up ProjectStatistics API. Passing result from $stats->generate() to $stats->getTotalRow() feels really weird. It suggests that we are missing an abstraction of "ComponentAnalyser". We can take a look into that once the API stabilizes.

[stefanzweifel]

Solid work. I definitely agree, that the $stats->getTotalRow() thing was kinda ugly.
I really like the components() and other() methods. In order to get #21 forward, we will have to sort component in 2 "meta-groups" (application-code and test-code). I think this could happen here (or in a new class and the ProjectStatistics class gets the data from the new class)