Refactor in smaller classes/modules Backend.lib

[MalloZup]

At moment the class Backend.rb has to many responsabilities.

We need to split it, in more indipendant classes. (or modules)

• TestExecutor class ( where all test execution operation are done)

• StatusPr class (where we handle all status operation)

• Tasks:

• [x] https://github.com/openSUSE/gitarro/issues/54 remove changelog

• [x] #67

• [x] 3

[juliogonzalez]

Given all the problems we are having with rubocop complaining about Backend.lib, we should consider the refactor a priority.