Add huge mushrooms and simple tree generator

This is corresponding to PR splitting as requested in #374.

This adds a complete huge mushroom generator. The old tree generator glowstone used is moved to TreeGenerator but it still the same.

Tree generation from GlowWorld.generateTree() method calls the StructureGrowEvent as it is expected.

#### Todo
- [x] rewrite a better generic tree generator (generates oak tree, small jungle trees and cocoa trees)
- [x] add birch / tall birch tree generator
- [x] add spruce tree generator

#### Test Plugin
- source avalable here: https://gist.github.com/jimmikaelkael/3d3448223c64870bbecd
- jar: https://drive.google.com/file/d/0B0gnjB7PN1h3SlEwZDFMdzdlV1U/view?usp=sharing