Fixed Small bug where scripts were being removed when they shouldn't be.

Scripts should only be removed when the item is being moved to another cell, otherwise they should remain active.
Implemented OnPCEquip special variable
allow OnPCEquip special variable to be of any type
Implemented OnPCDrop special variable

Scripts are responsible for resetting to 0, as investigation showed that is how vanilla handled it.
Unequipping items will reset OnPCEquip variable
Implemented OnPCAdd special variable

Had to edit OpAddItem in miscextensions.cpp, as local variables were not being initialised
for items added through it.
Does not get reset on drop, as per original morrowind.
updated changelog
rewrote the opencs cmake scripts (more compact and no more annoying warnings)
fixes for static build without cg
refactored special variable code
silenced a cmake warning
Merge branch 'cgfix'
Removed problematic ModVertexAlpha method, now done in shader
extern/shiny update: made caching more robust
cloud blending now as before (0 -> 0.25 -> 1)
Fix terrain derived data update (bug 534, bug 544)
better place for syncing
Merge branch 'master' of git://github.com/zinnschlag/openmw into terrainfix
updated changelog for 0.21.0 again
adjusted gcc settings again to avoid problems with older compiler versions