World creation needs a "force reset" flag

[DaveyBiggers]

As an optimisation, Malmo won't recreate the Minecraft world unless it decides it's not suitable for the requested mission. The result is that changes introduced in one mission will carry over to the next. In many cases this is a non-issue (eg when the agent makes no changes to the environment), but there will often be situations where a full reset is desired between missions.

We should add a flag to the XML for:

• FlatWorldGenerator
• FileWorldGenerator
• DefaultWorldGenerator
  to force world recreation.

[Implementation details: Flat/File/DefaultWorldGeneratorImplementation.shouldCreateWorld() should return true if this flag is set.]

[katja-hofmann]

Should also clear the inventory

[DaveyBiggers]

Fixed in #85.