tools, harness: support new YAML frontmatter

This supersedes tc39/test262 PR #46, #48, #52 and adds additional code to make packager.py work with the new YAML-based frontmatter.

parseTestRecord: add support for YAML frontmatter
parseTestRecord: initial unit test for test record parser
parseTestRecord: refactor for testing

factor old parsing; add YAML parsing

runner: support "includes" from YAML frontmatter

support frontmatter "includes" in python runner
use test.includes if present instead of scanning test code with regex

harness: factor individual functions out into files

tools: handle YAML errors

tolerate missing keys in dictionary (flags, includes)
report filename when empty frontmatter block
new option --list-includes to test262.py

harness: factor helper functions into separate files

sth: remove extra close-paren (syntax error)

test_common: TDD; failing parse of YAML

common: use parseTestRecord (YAML-aware)