Chdir into notebook's directory at kernel startup

This makes the behaviour consistent with how ipython handles notebook instances and avoid unexpected errors when using local files or modules.

Maybe there is a cleaner way to do this using the [path argument of start_kernel](http://ipython.org/ipython-doc/dev/api/generated/IPython.html.services.kernels.kernelmanager.html?highlight=mappingkernelmanager#IPython.html.services.kernels.kernelmanager.MappingKernelManager.start_kernel).