#23 Build warnings

[John D. Gwinner]

I just downloaded the library, followed the README to build it, and got a few warnings. These look like real bugs, but my guess is the two functions usually don't expose a valuable return code (well, how could they?)

/home/pi/raspicam-0.1.6/src/private_still/private_still_impl.cpp: In member function ‘int raspicam::_private::Private_Impl_Still::startCapture(raspicam::_private::imageTakenCallback, unsigned char*, unsigned int, unsigned int)’:
/home/pi/raspicam-0.1.6/src/private_still/private_still_impl.cpp:402:9: warning: no return statement in function returning non-void [-Wreturn-type]
         }
         ^
/home/pi/raspicam-0.1.6/src/private_still/private_still_impl.cpp: In member function ‘void raspicam::_private::Private_Impl_Still::commitBrightness()’:
/home/pi/raspicam-0.1.6/src/private_still/private_still_impl.cpp:605:17: warning: narrowing conversion of ‘((raspicam::_private::Private_Impl_Still*)this)->raspicam::_private::Private_Impl_Still::brightness’ from ‘unsigned int’ to ‘int32_t {aka int}’ inside { } [-Wnarrowing]
                 brightness, 100
                 ^~~~~~~~~~
/home/pi/raspicam-0.1.6/src/private_still/private_still_impl.cpp: In member function ‘MMAL_PARAM_IMAGEFX_T raspicam::_private::Private_Impl_Still::convertImageEffect(raspicam::RASPICAM_IMAGE_EFFECT)’:
/home/pi/raspicam-0.1.6/src/private_still/private_still_impl.cpp:820:9: warning: control reaches end of non-void function [-Wreturn-type]
         }
         ^