Added Support for GenerateStructures

We now check whether structure generation is allowed. This can be
configured on servers and when creating single-player worlds. If set to
false, Rift Gateways will not generate. Rift clusters still generate.
That was an intentional feature - we could make the setting disable
those as well if we want.
Merge pull request #58 from SenseiKiwi/master

Added Support for GenerateStructures