Added individual exp rates per weapon config

Server admins can set individual multipliers for each weapon in
sourcemod/configs/smrpg/weapon_experience.cfg. You can specify the
experience on damage and kill with the weapon.

Plugin authors can check the experience value with
SMRPG_GetWeaponExperience.
Updated cstrike exp module to use new native

The Counter-Strike experience module should check the
weapon_experience.cfg config too.
This makes the seperate knife damage convar obsolete.
Fix not properly ignoring "weapon_" prefix

The lookup for weapon experience could have failed when passing the
weapon classname with and without the "weapon_" prefix.