Rename data for unit tests
Add unit tests and data for CountBacklogTickets ITS function

Add unit tests and data about the issues_log table to calculate the
backlog of 'Open' + 'Reoponed' issues monthly and weekly.
Add R script to run the unit tests
Code refactoring for functions that complete with zeros

Two functions have been added to get the weeks and months between two
intervals of time. Those functions have been extracted from
completeZeroPeriod* and now are using also by other functions.
GetEvolBacklogTickets functions refactored

The function to the get evolution of the backlog by month or week
has been improved. Now it uses generic functions to the get the
periods (months or weeks) and accepts a list of statuses in order
to get the backlog for them.
Merge pull request #27 from VizGrimoire/backlog_of_issues_26

Code refactoring and tests for calculating the backlog of issues