Do not allow audio playback unless DSP is running.

Closes #122.
Add basic network streaming of audio over UDP.

The audio output of the receiver can now be streamed to a spefied host and
port using UDP. Currently only one channel is streamed using 48 kHz sample
rate. The audio is converted to short (S16LE) and a client side command
line could be:

  nc -l -u 7355 | aplay -r 48k -f S16_LE -t raw -c 1
Better color gradient for the waterfall.
Add icon for network button.
Small adjustment of default FFT colors.
Implement remote control interface.

This change set implements a hamlib/rigtctld compatible control interface
that allows setting and reading the current frequency. This is a simple TCP
socket using plain text commands and can be tested using telnet. For the
time being only connections from localhost and on port 7356 are accepted.
Move AFSK decoder to Tools menu.
Connect remote control activation to menubar/toolbar action.
Updated.
Added remote control configurator.
Fix segfault on exit with unintialized TCP socket.
Clean up leftover debug code.
Updated credits.
Update devices and docs.
Fix for division by zero error

Changing the gain on a TVRX connected to USRP1 was causing a floating point exception.
This fix prevents a division by zero error.
Merge pull request #129 from dtmf/patch-1

Fix for division by zero error
Updated.
Implement satellite event triggers through remote control.

This commit adds two new gqrx specific commands to the TCP control
interface and connects them to audio recording triggers.

The AOS command means acquistion of signal and will start the audio
recorder (unless it's already running).

The LOS command means loss of signals and will stop an ongoing audio
recording. It will do nothing if no audio recording is in progress.

These two event triggers are being added to gpredict and should be
available shortly.
Don't clear FFT buffer.

By leaving the already used samples in the buffer we ensure that there
is always enough data to perform an FFT of any size at any rate. While
this may give a scientifically less accurate FFT, it gives a much better
user experience.
Increase audio FFT sized and rate.