Expose explicit api

[thomasrudin]

Current api surface (stable):

xp_redo.get_rank(xp) : returns rank definition
xp_redo.get_xp(playername) : returns xp-value
xp_redo.add_xp(playername, xp) : adds or subtracts xp points

Less stable:

xp_redo.ranks : List with rank-defs
xp_redo.register_rank(rank-def) : adds a rank

Rank-Definition:

{
	name="Private Second Class",
	icon="xp_rank_01_private_second.png",
	color={r=255, g=245, b=245},
	xp=10
}