Adding runs differences info

The patch solved #17, #18 and #19 and contains:
- A new section in the web UI to display the differences
- A new PHP CLI script to compare runs
- New settings to set up thresholds to skip false failures
- Refactored code to avoid exitcode checkings so everything
  works as expected with set -e
Merge pull request #17 from dmonllao/MOODLE_25_STABLE

Adding differences detection