Add a support for screenshot with Cucumber

**What it's the actual situation on ARA ?**
Including screenshots in Cucumber Reports in a pretty easy to set up feature, while on ARA it seems to be a little more complicated, having to manage our own image files storage system.

**Describe the solution you'd like**
At least a rich documentation with detailed example on how to implement it
If possible, an API from ARA to be able to manage images files and having them link to reports (differently from the main DB)