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.

#### Test Plugin
- source avalable here: https://gist.github.com/jimmikaelkael/3d3448223c64870bbecd
- jar: https://drive.google.com/file/d/0B0gnjB7PN1h3UDYtSkJ5UDJFdVU/view?usp=sharing

I guess the code below is causing the bad blocks count in StructureGrowEvent.