Assigning names to constants

By assigning names to the constants, custom layout functions will be
able to easily determine the string equivalent. After all, .LOG_LEVEL
is not accessible outside the package namespace.
Changing constants to integers

No reason for them to be numeric, no floating point operations will be
used. And this might be a little more efficient.
Change layout.simple to use level names

Instead of relying on .LOG_LEVEL, use the level name. This is simpler
and can be used by custom layout functions built by users.
Merge pull request #3 from asieira/constants

Small improvement to constants