[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.
date_from and date_to are not aware of timezone so I thought to convert them as we made for the attendances.