Implement nether portals and their mechanics

[JustTalDevelops]

This is a slightly tricky issue, because traditionally the code used for portals, specifically nether portals, is quite... awful, and creating an API that fits the rest of Dragonfly's design philosophies is also going to be a bit of a design challenge. However, there are plenty of clever approaches that can be taken, and after a few of the bigger hurdles, it is generally quite simple. A couple notes I took a little while back:

• The vanilla portal implementation scans first on the west/east directions, and then the south/north directions.
• The maximum portal width and height, including the frame, is 23 blocks.
• The minimum portal width is 4 blocks, and the minimum height is 5 blocks, both including the frame.

There are a few decent implementations worth looking into, such as MiNET's or the work-in-progress GlowstoneMC implementation.

I think most of all, we should try to aim for:

• A fluid API to make it easy to utilize portals in code.
• Vanilla functionality.
• Minimal code. (this is where plenty of other implementations fail, for example, Nukkit's.)