rake: use yard for the api doc
test: set rspec options

* use `--format documentation`
* use `--color`
rake: add `rake test`
social_snippet.rb: use require_relative() instead of require()
spec_helper: require local modules
spec: add tag_parser_spec
spec: add tag_spec
tag_parser: add tag_parser.rb
tag: add tag.rb
tag: add constructor
tag: add Tag.is_tag_line()
test: add tag_test.rb
tag: small changes

* rewrite initialize()
  * to use class method
* remove is_tag_line()
  * to be is_snip_tag_line() / is_snippet_tag_line()
spec/test: use byebug and debugger to debug

* use byebug
* use debugger (ruby version < 2.0.0)
tag: add is_snip_tag_line() / is_snippet_tag_line()

* to find @snip/@snippet tag
tag: add is_snip_or_snippet_tag_line()
tag: add is_tag_line_with_repository()
tag: add get_repo()
tag: add get_path()
tag: add get_prefix()