Raspistill: Add support for requesting camera settings (--settings)

New option that asks the GPU to send events whenever the camera
settings parameter changes. That contains exposure, analog and
digital gains, AWB gains, and focus position.
Change the requested frame rate on longer exposure times

Allows selection of the longer exposure sensor modes.
Merge pull request #189 from 6by9/PR-20140725

Raspistill options for camera settings, and for long exposure modes