World Seed

The WorldContainer class now has a method for turning nullable strings into a numerical seed for the world.  If performs the following steps/checks:

1. If the nullable string is null, it uses the cryptographically secure PRNG to obtain a seed.
2. If the nullable string represents an integer which can be represented by an unsigned 64-bit integer, it uses that integer.
3. It applies djb2 to the string to obtain a seed.

Also added some comments to world.hpp and updated Makefile.