Lots of fixes

Changed all instances of self.Entity (deprecated) to self. Changed ATM
machine error to be more generic. Can now deposit/withdraw below £1 in
ATM (fixes #57). Changed all instances of TheOwner to Owner (fixes #48).
Added utility functions ent:SetSharedOwner/ent:SetWorldOwner and changed
everything to use them. Fixed cash spawning with no set amount.
Refactored weapon_keys. Refactored police door system (fixes #49).
Pressing R with keys now only has a 1 second cooldown (previously 3).
Improved rp_downtown_v2 map file, fixing a pesky door that moved after
spawn. Refactored cl_changeteam. Remade cl_playeruse menu to be static
(fixes #56). Fixed a bug in cl_playeruse where entering nothing into the
box and pressing a button gave strange output. Added drug prices (fixes
#50). Removed unneeded sh_holdfix (issue was fixed in GMod itself).
Changed citizen to be default class (fixes #58). Mayor is now marked as
police (for police doors, teamkill, etc). Heavily reduced code in
sv_startup (could be improved further however). Class weapons now spawn
before default weapons (fixes #63). Changed '!' to '.' in output from
!dropcash and !setcash. Fixed not being able to drop cash values
containing decimals in !dropcash and !setcash (fixes #64, fixes #65).
Money now has shared prop protection by default (fixes #66).