Added steering centre members to VehicleData.

steeringReturn is the main control for the amount of return in
the steering. Setting it to 0 (the default) disables steering return
completely.

steeringReturnSpeedScale is a multiplier on the amount of effect that
the vehicle's velocity has on its steering return.

powerSteering can be set to eliminate the effect of steering return on
user input. Without this flag set, the user will have a harder time
steering, because the wheels try to return as they steer. If this flag
is set, steering return will not be applied while the user is actually
steering, only when they release.
Added steering return to Vehicle.

It happens in Vehicle::updateMove so it can apply to all vehicle
subclasses. It is applied to both x and y steering.
Fixed stream read/write mismatch bug.

It was causing Vehicles to roll forwards with a constant velocity when
stationary, and ignore move->y.
Added compilation information to README.md
Updated readme with PhysX 2.8.4.6 information.
Consider zoom when culling billboards.

screenScale represents the current zoom factor, but may need some
revision because I'm not sure if what it's doing is technically correct.
It is used to multiply the culling distance for cells, as well as the
fading distance.
Added comment.
Updated logic in MissionArea::getServerObject.

Before this function would iterate through every server-side object to
find a MissionArea. Now, it stores the server object in a static member
smServerObject. This member is updated in onAdd and onRemove, and returned
by getServerObject.
Fix for Issue #155 for Billboard Creation
Merge pull request #156 from DavidWyand-GG/issue155-BillboardCreation

Fix for Issue #155 for Billboard Creation
Partial revert of changes from Pull #125

Pull Request #125 was to fix Issue #124.  However, as reported by a
user, this fix broke the water.  This is a partial reversion of those
changes to get the water working again.  The fix from Pull #125 will
need to be examined.
Merge pull request #158 from DavidWyand-GG/issue124-RevertMaterialChanges

Partial revert of changes from Pull #125
Fix for Issue #166 for Client Not Loading

The issue was with sending the ack mask to the client when the ack byte
count was 4.
Merge pull request #167 from DavidWyand-GG/issue166-ClientNotLoading

Fix for Issue #166 for Client Not Loading
Fix for Issue #170 Asset loading under Linux
Merge pull request #171 from DavidWyand-GG/issue170-AssetLoadLinux

Fix for Issue #170 Asset loading under Linux
Fix for Issue #172 for Turret Crash on Dedicated Server

The Cheetah's turret fire sound datablock happened to be named the same
as the player's turret weapon's datablock.  This caused the sound
profile's description to be NULL on the dedicated server.
Merge pull request #173 from DavidWyand-GG/issue172-DedicatedTurretCrash

Fix for Issue #172 for Turret Crash on Dedicated Server
Fix for Issue 174 for Turret Targeting Wrong Direction
Merge pull request #175 from DavidWyand-GG/issue174-TurretTargetWrong

Fix for Issue 174 for Turret Targeting Wrong Direction