Working on impropving plugins by standardizing report. See #81.

Currently, we are working on making report more sensible by returning
consistent report format. This commit attempts to fix alive plugin.
Most of the discussion happen on this pad:
https://etherpad.mozilla.org/3F4KSao89V
Make XFrameOptionsPlugin returns consistent report. See #81.

This commit copies and pastes a lot of code. Next commit should attempt
refactor some common code.
Added _format_report to AbstractPlugin.See #81; Refactored alive report.

_format_report(..) can help us reduced lines needed to format a standard
report. This commit only fixed alive plugin.
Refactored XFrameOptionsPlugin to use _format_report. See #81.
Standardized HSTSPlugin output. See #81.
Standardized XContentTypeOptionsPlugin. See #81.
Standardized XXSSProtectionPlugin report output. See #81.
Standardized ServerDetailsPlugin report output. See #81.

The test is currently disabled.
Standardized RobotsPlugin report output. See #81.
Fixed #81. Completed Standardizing report output for all basic plugins.

In test_api.py, we actually can see all 8 plugins emitting issues and we
have hardened the test_api.py checking the issues.
Manually merged with 981e42b6b1
Merge pull request #91 from yeukhon/master

Fixed #81. Completed standardizing report output for all basic plugins.