levels: map15 2014-01-06 update from Z0k

-Added more medikits only for skill Easy and Medium.

-Moved the Supershotgun that can be found in the left outside part of
the base, now its moved where the 2 chaingunners came down from the
lift.

-Moved the secret rocket launcher where the supershotgun was found
before, this because i consider rocket launcher being helpful for this
map and keeping in secret (in a secret lift where the 2 chaingunners
came down from the lift) was kinda hard for some players.

-Added Plasmagun as secret, can be found now where the rocket launcher
was found previously.

-Added BFG9000 as secret, can be only found on the secret exit.

-Added Megasphere as secret, can be only found on the right side of the
secret wall where some enemies came out.

-Change the texture of the secret walls near of the exit, which such
walls lead to the secret teleporter for the secret exit.

-Made some changes to enemies, and added back the "hard part" of the map
only for skill 4 (hard) this mean:

*the hell knight which teleported is back.
*chaingunners are back on the red key zone
*revenant is back on the left side of the base where player can find
*the rocket launcher.
*hell knight pair is back after players hit the swicht on the toxic
tower zone and head back to main zone.
*chaingunners at start are back.
*chaingunners near of the imp are back.
*Revenant on the small storage room is back.
*hell knight where player plays with the swicht (and can find the secret
swicht) is back
Only those monsters will appear on Skill4 (hard) as Cato pointed out.
newbs shouldnt be playing harder difficult skill at start.

-made some changes on enemy flags: some monsters will appear on skill
1-3.

DW: http://www.doomworld.com/vb/post/1228815
Signed-off-by: RjY <rjy@users.sourceforge.net>