Update session.c

Players with Overall Level 179 won't gain any OA exp anymore, so the stats window will show exp/min 0.0.

Instead of printing out the difference OA exp directly, the experience of all skills will get summed up in a loop (L147) and used for displaying exp/min instead (L165).
Update session.c

Removed unnecessary blank line.
Merge pull request #13 from Adamantinus/session_exp_per_min

Session exp per min