Do not use strings in VesselState gimbalExtDic

The dictionary now uses System.Type as identifiers.
Keys are PartModules types (ModuleGimbal), or object type for null
gimball.

Initialization is also moved into the static constructor as it is not
related to the object's instance.
Merge branch 'dev' of https://github.com/MuMech/MechJeb2 into performance

Conflicts:
	MechJeb2/VesselState.cs
Merge pull request #493 from xytovl/performance

change key in gimbalExtDic for a minor performance improvement