Make header syntax consistent about php-reports

[jdorn]

Done:

• Implement new format for almost all headers (still need VARIABLE and a few others)
• Consistent header validation
• Documentation references the new format and contains mostly JSON examples

Todo:

• Make json parsing less strict
• Update debug report format
• Convert VARIABLE header to new format and update parameter names

[jdorn]

Single quotes, unquoted keys, and trailing commas are supported now in JSON headers. The following is now valid:

HEADERNAME: {
   'key': "value",
   key2: 'value2',
}

The variable header is converted over.

I'm going to make a separate issue for updating the debug_report. There's a lot of improvements that can be made in addition to before/after parsing headers.