Add cmake module for finding udev.

This adds a cmake module to find udev/libudev for linux.
Add udev helper library to v4l2 plugin.

This adds a small helper library to the v4l2 plugin that uses udev to
get events for v4l2 devices.
Use udev events in the v4l2 plugin.

This adds code to set up the udev monitoring library and use the events
to detect connects/disconnects of devices.
When the currently used device is disconnected the plugin will stop
recording and clean up, so that the device node is freed up.
On reconnection of the device the plugin will use the event to
automatically start the capture again.
Fix left open file descriptor in v4l2 plugin.
Merge pull request #284 from fryshorts/v4l2-input

Add udev support to v4l2 plugin.