Added option to skip the prefix of temp files with file:///

[bo-oz]

Made two changes:

1/ Added option option[:slip_file_prefix] that prevents pdf_from_string from calling .pdf_from_html_file and calls pdf_from_url directly.
2/ This option also influences the temp file that is created for header/footer content
3/ Passed the options[:temp_path] to the Tempfile that is being created for the header/footer content.

[unixmonkey]

@bo-oz Why is this desirable?