[FIX] hr_attendance_analysis various bugs

This PR contains the changes made on https://code.launchpad.net/~agilebg/hr-timesheet/fix-hr_attendance_analysis-various-bugs before the project were hosted on github.

Main changes
-------------------
1. Merged branch lp:~agilebg/hr-timesheet/fix-hr_attendance_analysis-reference-is-not-defined and moved the related MP https://code.launchpad.net/~agilebg/hr-timesheet/fix-hr_attendance_analysis-reference-is-not-defined/+merge/221250
2. Merged branch lp:~agilebg/hr-timesheet/fix-hr_attendance_analysis-bracket and moved the related MP https://code.launchpad.net/~agilebg/hr-timesheet/fix-hr_attendance_analysis-bracket/+merge/221278
2. Merged branch lp:~agilebg/hr-timesheet/fix-hr_attendance_analysis-report-timezone and moved the related MP https://code.launchpad.net/~agilebg/hr-timesheet/fix-hr_attendance_analysis-report-timezone/+merge/221374
3. Merged branch lp:~agilebg/hr-timesheet/hr_attendance_analysis-imp-time_difference-message and moved the related MP https://code.launchpad.net/~agilebg/hr-timesheet/hr_attendance_analysis-imp-time_difference-message/+merge/225275
4. Deleted some unused methods and applied a bit of refactoring
5. Fixed the total leaves calculation: now the current_total_leaves value is calculated considering the working time and the timezone

According to https://github.com/OCA/hr-timesheet/issues/3  as soon as the PEP8 cleanup will be released, in case I'll manually resolve the conflicts.

Thank you.
After investiganting, I saw the working schedule is not converted to timezone. That is, if your working schedule is from 09:00 to 13:00 (as you see it in the web interface), in the DB the values are exactly 9 and 13.
On the other hand, the leaves (as well as the attendances) are always saved as UTC in the DB. That is, if you leave at 09:00 and your timezone is GTM+2, the DB will contain 07:00.
So, the conversion here is correct
:+1: