Added stub of peak-detection feature
Added UI for peak detection
Documentation
Fixes, cleanup and peak snapping
Add sample rates for BladeRF and Netsdr.

Patch from Dimitri Stolnikov.
Merge pull request #121 from Steven--/peak-detection

Adds peak detection to FFT plot.
Updated.
Merge branch 'master' into peak-hold

Conflicts:
	applications/gqrx/mainwindow.cpp
	applications/gqrx/mainwindow.h
	qtgui/dockfft.cpp
	qtgui/dockfft.h
	qtgui/dockfft.ui
	qtgui/plotter.cpp
	qtgui/plotter.h