Scout movement weapon

We were pretty happy with this in the old beta; just needed to get it finalized. Cherry-picked all the relevant commits from the dev/svn branch (hopefully I didn't miss any).

Things left to do:
- [ ] Name it (its currently code named jumpdown because I'm a dumbo)
- [ ] Give it a weapon model (currently uses the railgun model)
- [ ] Give it a unique sound (currently uses the railgun sound)
- [ ] Give it a better effect (currently uses a simple conc-like ring)

For anyone unfamiliar:
* When used, gives the scout a double-jump-like boost (moves them in the direction they are looking, not an additive boost)
* Must be fully charged to be used
* Charges passively while held, loses all charge when switching weapons

Some quotes of mine from the old beta forums:

> The goal is to communicate that the scout is primarily for movement. The current (2.4) weapon loadout does the exact opposite.

> So, we basically have two options:

> - Remove all weapons, leaving the scout with "hands"
- Add a movement weapon that requires you to have it out basically at all times except for certain cases (nailgunning an SG for instance)

> We have done the latter, and I think it's a far better solution.

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> The other thing is that it's not really meant to be the most useful thing ever. As long as it is used instead of damage weapons, the goal has been met.

> The scout is for movement, that is all we want to communicate. If this weapon does that, then fantastic. 

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> Anyone playing scout trying to get kills is not going to be successful, and will conclude (incorrectly) that the game is unbalanced because of the scout's perceived weakness. This movement weapon will just make sure that no longer happens; if people no longer play scout, that's their prerogative. At least we'll no longer see D scouts.

> Also, scout has always been a "forced" role (in that his weapons are awful; the best scouts don't use weapons). We're just making sure people understand that role, because as of now, it's not easy to grasp with the clues FF gives (it's hard to tell that his weapons are intended to be awful, and that it's better if you avoid using them altogether). 

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> If this gets any one person to think "oh, I shouldn't be killing people with this class", it will have succeeded. If it also doesn't unbalance the scout in CTF while at the same it time improves the scout's usefulness in AvD, it will have succeeded quadrupaley.

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> Here are the goals I personally had in mind for a scout movement weapon:

> * Simple, accessible movement
* Unique new movement tool (opens up new possibilities with movement, not just adding another way to achieve similar movement)
* Not overpowering (or mostly doesn't affect the balance of the scout)
* Can only be used for movement
* Requires that it be used in place of damage weapons
* Communicates that the scout shouldn't use damage weapons
when we are happy with the jumpgun behaviour we can remove a lot of this testing-different-velocity-maths stuff (I suspect the time might be now).