do not modify parameters unless explicitly set

The contrast, saturation, brightness, sharpness and focus parameters
were recently added to usb_cam. This caused a regression
(sigproc/robotic_surgery#17) whereby the default settings for a webcam
are overridden in all cases by the hard-coded defaults in usb_cam.

In the absence of a know good set of "default" values, leave the
parameters unset unless the user has explicitly set them in the launch
file.
Merge pull request #24 from rjw57/do-not-touch-parameters-unless-asked

do not modify parameters unless explicitly set
Update README.md
Warn rather than error if framerate can't be set

The driver doesn't currently work with em28xx based devices as they don't allow the framerate to be set directly and the node exits with an error. Changing to a warning allows these devices to be used.
Merge pull request #25 from blutack/patch-1

Warn rather than error if framerate can't be set
changelog updated
0.1.13