Add chance attribute to blockdrops.

[Pugzy]

I found most of these issues whilst testing #1039 so I thought I'd fix them too.

This class could do with a re-write to enable the punch and trample elements in the future.

• Add chances to item drops.
• Gather wrongtool info when attribute and element.
• Fix usage of wrongtool.
• Fix giving incorrect experience.

Update 1:

• Use new parsing.
• Gather reset of elements.

Update 2:

• Can't remember how on earth this stuff works.
• Create new method for generateDrops() to be used in all events.
• Start support for trample and punch.