[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
2. Merged branch lp:~agilebg/hr-timesheet/fix-hr_attendance_analysis-bracket
2. Merged branch lp:~agilebg/hr-timesheet/fix-hr_attendance_analysis-report-timezone
3. Merged branch lp:~agilebg/hr-timesheet/hr_attendance_analysis-imp-time_difference-message
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 he PEP8 cleanup will be released, in case I'll manually manage the conflicts.

Thank you.