Add Debug-Mode to Raven

[kaylarose]

Add a simple way to put Raven in "Debug Mode" - basically a way prevent it from sending messages to the server - but still allow Raven to handle exceptions and messages as-normal.

This prevents exceptions/messages during Development Cycles from cluttering up the Sentry stream - as well as being able to debug values for Messages that are sent to Raven.

Example Usage:

    [RavenClient clientWithDSN:@"XYZ"];
#ifdef DEBUG
    [RavenClient sharedClient].debugMode = YES;
#endif
    [[RavenClient sharedClient] setupExceptionHandler];

NSLog Output:

Sentry JSON (will not be sent in Debug Mode):
[JSON BODY]

[asbjornu]

While I don't oppose this change, I do think you would be better off having different DSNs for each environment you deploy your application to. We, for instance, have one for development, one for staging and one for production. Then you can also set up different alert policies on each project, so you only get e-mail on production errors, for instance.

To group each project (environment) into a common bundle, you can create a "Team" that holds the name of the application.

[dcramer]

We enable this behavior on other platforms simply by not configuring a DSN. 

[timorzadir]

For the sake of consistency, I think we should enable this feature by not configuring a DSN as suggested by @dcramer.