Set Content-Disposition in RandomPage HTTP Helper

Fixes: https://github.com/TheTorProject/ooni-probe/issues/305
Verify the sha 256 sum of the zlib package.
Escape all strings before writing them to the log file.

This fixes: https://github.com/TheTorProject/ooni-probe/issues/302 for the backend
Merge pull request #43 from TheTorProject/fix/issue_g

Set Content-Disposition in RandomPage HTTP Helper
Merge pull request #44 from TheTorProject/fix/issue_d

Verify the sha 256 sum of the zlib package.
Merge pull request #45 from TheTorProject/fix/issue_c

Escape all strings before writing them to the log file.
Refactoring and tidying up of various different parts of the codebase.

Let pylint guide the way!
Add unittests for the report handlers.

Add a useful base class for implementing any unittest.