Change SMRPG_AddClientExperience to include reason

Plugins can now set the reason why they give someone experience.
SMRPG_OnAddExperience includes that reason now and is fired for all
calls to SMRPG_AddClientExperience instead of only for events in the
core.
Added more control over bot experience

You can set if bots get experience for interacting with other bots or
players and if players get experience for interacting with bots.

In the Counter-Strike module, you can set if bots should get experience
for completing game specific objectives.