Expose the handled property to users

[cleptric]

One of the most common ways frameworks integrate Sentry is by using captureException somewhere in the framework-specific error handling code. This creates a small issue though, as everything that gets sent via captureException is handled: true, and only errors handled by the SDK's error handler are handled: false.

One solution to this could be the addition of a new function, \Sentry\captureUnhandledException, which would allow people to mark exceptions as handled: false.