Added unit timeout settings value.

The unit timeout determines after how much that the information from moused
over units should be purged.  This is being set to twenty minutes for now, as
that should be well beyond the amount of time before any sources despawn and
thus can not be looted.
Added function PurgeOldMouseoverUnits.

This function removes mouseover units who both have a last mouseover time and
have their last mouseover time older than the unit timeout setting.
Register the LOOT_CLOSED event.
Purge old mouseover units when the loot window is closed.
Added function LootGain_PrintGlobalDataShort.

This function simply prints the name of each source and the date when it was
recorded.
Moved setting default global variables to function.

Instead of setting default (re: initial) values of LootGain_Data and
LootGain_CharacterData outside of any function, these are now set in a new
function LoadSavedVariables, which checks if the saved variables exist.  If
they do not, they are initialized to default (empty) values).
Added Initialize function.

This function does not do anything meaningful.  It simply prints a message and
sets the initialized variable to true.
Register ADDON_LOADED and PLAYER_ENTERING_WORLD events.
Set LootGain.initialized with a default false value.
Do not initialize mouseover units when file is executed.

This is now done when the ADDON_LOADED event is fired.
Add handlers for ADDON_LOADED and PLAYER_ENTERING_WORLD.
Added saved variables.

There is one variable that is saved for all characters, LootGain_Data, and
one variable that is saved for each character, LootGain_CharacterData.