Add named colors support and automatic loading in database. Also fix some problems.

[NEW] Now can trap the error in coroutines.
[NEW] Add nexus.core.database.getColor().
[NEW] Add nexus.core.graphics.clear() to clear the color setting.
[NEW] Now nexus.core.database.initialize() will take parameters to
automatice load databases.
[NEW] New color.lua in database which defining named colors.
[MOD] Rename nexus.core.*.clear() to reset().
[MOD] Now nexus.core.database.loadTextData() will auto change the
translation tables.
[MOD] Move constants in conf.lua to bootstrap.lua file.
[MOD] Move lua extendsion to bootstrap.lua file.
[FIX] Fixed a typo-error in game.lua file.
[DEL] Remove the usage of love.graphics.setColorMode() and
love.graphics.getColorMode().