New certificate schema, mandatory type, chart sha and timestamp

[mmulholla]

This PR covers several things

1. New schema implemented. See "Certificate" struct in chartverifier/Certificate.go
2. Added ability to mark checks as Mandatory, Optional or Experimental. All current checks are mandatory
3. Generating a sha256 value for the chart. Basically sort all files into alphabetic order and then create sha from their content. Test case repeatedly creates sha calues for test charts making sure they are always the same.
4. Report is output to console and no longer a file. Running locallly use "command 2> report.yaml", running docker image just "command > report.yaml"
5. Default output is now yaml.
6. Some tests had to be updated for the new changes and schema

[mmulholla]

@isutton @pedjak Some of the comments relate to use of certificate instead of report. I used certificate because it is consistent with the rest of the code. The schema is in Cerificate.go, it is created from certificatebuilder.go, the check process is created by certifierbuilder.go and run by certifier.go. If we use Report for the schema we should change all of those, the object and funcs too. Thoughts?

[pedjak]

If we use Report for the schema we should change all of those, the object and funcs too. Thoughts?

Yes, change everything what is needed.

[mmulholla]

@pedjak @isutton : have addressed the comments which are not connected to using report instead of certificate - for those comments and the entire rename effort I opened a new issue #78

all chnages have been merged or deferred to #78