Added optional protocol method that is called when camera source is loaded

This is a small commit related to issue #86 that I opened and subsequently closed. I added an optional protocol that is called when the camera source is loaded. This allows the developer to set initial camera settings by calling the VCSimpleSession methods after the camera is loaded from the delegate. 
No no what I mean is that if the user hasn't implemented the delegate method didAddCameraSource: then they will get a crash.  The `@optional ` keyword really just notifies the compiler not to throw an error if the method hasn't been implemented.  It's still up to the caller to check to see if it can be called first - unless something's changed since the last time I made some optional methods for a protocol.  You should just be able to do `if ( [_delegate respondsToSelector:@selector(didAddCameraSource:)]) { ... }`