Relative path support, referer header and more...

Added support to "relative paths" (function relative2absolute)
Added "referer header" (if exists)
Added "remove charset" for mime-types (eg. text/html; charset=ut8 => text/html)
Added prefix in files created by html2canvas-php-proxy
"remove_old_files function" removes only the files with prefix
Version 0.1.3 to 0.1.4