encoding is detected using the `mb_detect_encoding()`-function
in strict-mode. Checking is done before the parsing starts.
Without this patch, the calls to `preg_replace()` using the
'u'-modifier would cause the non-utf8-string being processed
to be empty when parsing starts which makes the parsing return
no result.
Added support for the full range of escaped values in double quoted
strings in chapter 5 of the YAML 1.1 and 1.2 specs. The escaping
and unescaping strategies were factored out into separate classes to
keep the logic isolated.
Added examples from the spec to the unit tests for all escaped values.
