Harmonize coding style and whitespaces
Remove workaround for signatureWithObjCTypes:

This was made public in 10.5.
Remove workaround for WebView setDrawsBackground:

We no longer support versions of Mac OS X where is this not available.

Also, updated usage string to remove mention of 10.5.
Update to new APIs for reading/writing plists

These do not cause a problem yet, but the new APIs follow proper Cocoa memory management rules and will make upgrading to ARC easier.

Also, since we now return an NSError instead of an NSString, we should not expect upon successful completion that these methods will return nil for the passed NSError object.
Use (Core) Foundation data types
camelCase local variables
Don't mix BOOL and bool
Use Objective-C literals and boxing
Use anonymous categories to declare private interfaces

Anonymous and named categories actual behave differently. Using the latter, prevents us from declaring (private) properties.
NSAutoReleasePool → @autoreleasepool
Update to ARC
Don't use the pre-processor to declare a string constant

Also, renamed DialogServerConnectionName → kDialogServerConnectionName. This is more consistent with the TextMate coding style.
Use proper exit codes
Remove debug statements
Remove include guard
Improve help message

I added a usage statement to the help message and did some refactoring to alphabetize the list of registered commands. Along with some minor fixes, such as removing the redundant "help" in the invocation string when running "$DIALOG" help help.