Added huge mushroom and more tree generation.

Includes a better generic tree system based on oak, small jungle, and
cocoa trees, as well as birch, tall birch, and spruce tree generation.
Now fires StructureGrowEvent during tree generation.
Added script that fetches Glowkit PRs for Travis.

For PRs, the PR body will be looked up and scanned for links to Glowkit
PRS, which will then be included in the build. Also reorganizes the Travis
script slightly.
Fixed PlayerMoveEvent handling and teleportation offset.

When PlayerMoveEvent's destination is changed, it should be considered a
teleport and fire the appropriate event and handle world changes.

PositionRotationMessage now refers to the client's feet instead of eyes.