[WIP] Add more tree generators

This PR is an extension to PR #430.
It adds the following trees:
 - Acacia
 - Dark Oak
 - Swamp Tree
 - Jungle bush
 - Big jungle tree
 - Tall redwood (those who have just a few leaves)
 - Mega redwood

After #430 is pulled and this PR rebased on it, the amount of code should be around 800 lines.