SP Ironsights #20, Tau Prediction Fix, SP Crossbow Bolts #21

[ampreeT]

Changes:

• Added feature SP_WEAPONS that controls whether to use single-player functionality for weapons
• Added ironsights for weapon_357 and weapon_crossbow (Controlled by feature SP_WEAPONS)
• Added regular crossbow bolts and changed crossbow skin (Controlled by feature SP_WEAPONS)
• Added tau cannon prediction fix (Issue was caused by hooking CWeapon_Tau::ItemPostFrame() and setting IsMultiplayer to false)

Fixes #20 and #21

[ampreeT]

Currently, the only issues are the following:

• Secondary attack on the crossbow does not fire from the center of the screen
• Secondary attack on the crossbow still plays mp sound
• Crossbow suffers from prediction issues caused by CWeapon_Crossbow::ItemPostFrame() being hooked, but this is needed currently for SP implementation

I'll make a new issue for the above once this gets merged in.