OWTF 0.40 "Summer Storm III": updates on the reporter modules

- Open files on unicode encoding
- Escape the transaction request and response, limit the nb of lines,
  convert it to unicode
- Pass the offset value to the plugin report template
OWTF 0.40 "Summer Storm III": update the styling of ResearchHeaders
OWTF 0.40 "Summer Storm III": update the template generated by some
plugin_helper functions