Allow fractional ppm correction

0.1ppm should be enough for anybody... O:-)

But seriously, both my hackrf and rtlsdr are something like X.4-X.6 ppm and
I really want my waterfall display to line up with my reference frequency.
take credit for my li'l diff
restore ppm correction from config
Merge pull request #201 from ckuethe/master

Allow fractional ppm correction.