Refactor to psutil #2, added tests

[rsiera]

Issue: https://github.com/zalando/pg_view/issues/60

Second part of refactoring to psutil. Includes tests and separating logic into classes which try not to break single object responsibility principle. This commit is preparation for last PR, which finally introduces psutil and makes pg_view platform independent.

[alexeyklyukin]

Hi @rsiera, thank you for the PR, I will take a look at it.

[a1exsh]

Wow, this is still a lot of changes. Unfortunately, it takes more time to review this than I can allocate currently.

[rsiera]

Hello guys, can we back to this PR and finish the review process of that, so I can fix/apply changes ? I'm aware that it's quite big PR but these tests should make the project more stable and editable. + it finally solves #60 ;)

[alexeyklyukin]

@rsiera sorry, yes, that's a lot of code. Let me try to give it another shot in the upcoming days.