Increase the transition width of the channel selection filter from 2000 to 50000 Hz.

This greatly reduces CPU usage, and doesn't appear to have any effect on the
performance of the receiver.  Without this change, the flowgraph can't even
keep up on my relatively powerful system (Core i5-2520M CPU @ 2.50GHz),
resulting in stuttering audio and RDS decoding errors.
Fix the FM de-emphasis sample rate. It should be 70 ksps, not 140 ksps.
Increase the default volume by 15 dB, since it's very quiet at present.
Add apps/rds_rx.py to .gitignore.
Correct the bandwidth of the DSBSC filter.
Increase the baseband rate to 240 kHz to prevent aliasing when demodulating
the L-R and RDS channels.
Replace the pilot detector with a PLL.

This reduces noise, and ensures that the pilot amplitude is equal
to one, so that the L-R signal is not attenuated.  Before this fix,
it was so low that stereo audio could not be heard.
Replace the RDS low-pass filter with an RRC filter.

According to this document, the transmitter uses an RRC filter with
alpha=1, so I assume the receiver would work best with a matched
RRC filter:

http://www.db-thueringen.de/servlets/DerivateServlet/Derivate-18898/54_IWK_2009_1_0_08.pdf
Merge pull request #1 from argilo/master

Various improvements to the sample GRC flowgraph