[FEATURE] Override User-Agent HTTP header

A new TypoScript setting "overrideUserAgent" is introduced that allows
overriding the default HTTP User-Agent header of wkthmltopdf.
[TASK] Fix line breaks in ext_conf_template.txt

The linebreaks are changed from DOS to UNIX.
[TASK] Add missing PHPDoc for cache manager in tx_webkitpdf_pi1
[FEATURE] Auto-create output directory if it does not exist

Since the typo3temp directory might get cleared from time to time the
target directory will now be automatically created to prevent errors.