Instrument all files from the beginning (fixes coverage results).

With this pull request, attester will instrument all files from the beginning instead of instrumenting them on the fly when they are requested by a browser.

This fixes the coverage results (especially: the global percentages of covered lines, functions and conditions).
Before this pull request, if any file was not requested at all, it was not included in coverage results, leading to overestimated global coverage percentages.
With this pull request, all files included in the coverage configuration are taken into account.

Before integrating this pull request:
- [x] Check that piuccio/node-coverage#26 is integrated and a corresponding version is available on npm
- [x] Update the version of node-coverage in package.json.