Renamed Core.read() to Core.load().

[NEW] Now extend love functions in bootstrap.lua.
[MOD] Renamed Core.read() to Core.load().
[DEL] Removed Core.exists().
[DEL] Removed original Core.load() and replaced with original
Core.read().
Added failed argument to love.filesystem.load().

[NEW] Added failed argument to love.filesystem.load() which can be
called when the load failed. Default function is error.
Added love.filesystem.loadChunk().

[NEW] Added love.filesystem.loadChunk().