Support for multiple milestones statistics summarization

1) Adding support to process one or more milestones.
2) Adding statistics to "Top time consuming issues"

#### Usage:
```
usage: client.py [-h] --user USER --passwd PASSWD --repo REPO --milestones
                 MILESTONES

Script used to generate a WebOps Sprint Report

optional arguments:
  -h, --help            show this help message and exit
  --user USER           User to login on GitHub
  --passwd PASSWD       Password to login on GitHub
  --repo REPO           Repository to be analyzed
  --milestones MILESTONES
                        Comma sepparated milestones to be analyzed

All that's well, ends well.
```
#### Example of usage:
```
python client.py --user frmaia --passwd passwd --repo sambatech/webops --milestones 82,83,84,85
Processing Issues to milestone Release 4 - Sprint 1 - 09/06 à 13/06
Processing Issues to milestone Release 4 - Sprint 2 - 16/06 à 20/06
Processing Issues to milestone Release 4 - Sprint 3 - 23/06 à 27/06
Processing Issues to milestone Release 4 - Sprint 4 - 30/06 à 04/07
=========== Milestone(s) Statistics ===========

........... Task Summary ...........
[ ... ]
........... Top time consuming issues ........... (NEW)
[ ... ]
```