Remove all isOp() checks and use Rank instead

[EmilHernvall]

There's a bunch of isOp references left from the old code:

src/info/tregmine/commands/NewSpawnCommand.java
18:        if (!player.isOp()) {

src/info/tregmine/commands/RegenerateChunkCommand.java
19:        if (!player.isOp()) {

src/info/tregmine/commands/FillCommand.java
140:                if (!player.isOp() &&
170:                if (!player.isOp() &&

src/info/tregmine/commands/QuitMessageCommand.java
137:        if (victim.isOp()) {

src/info/tregmine/api/PlayerDelegate.java
1117:    public boolean isOp()
1120:        return delegate.isOp();

src/info/tregmine/ChatHandler.java
277:                if (!session.isOpen()) {

src/info/tregmine/listeners/TregminePlayerListener.java
451:        if (!player.isOp()) {

src/info/tregmine/listeners/ZonePlayerListener.java
505:        if ("world_the_end".equals(dstWorld.getName()) && !player.isOp()) {

I'd like to see them use Rank and Player.getRank() instead, like we do everywhere else.