#556 Use GeSHi to display mail headers

[BenBE]

Since Version 1.0.8.4 GeSHi supports highlighting of RFC mail headers and raw maildir messages using the "email" language. This would significantly improve the display of headers as more details are marked than just plain header names vs. header values.

Due to some issues in 1.0.8.4 the more recent 1.0.8.5 should be used instead.