added code for chip_decoder

decoding does not work correctly yet - will commit onto
experimental branch until it does
trying out different things to make the decoder work

d_read should not be needed for the decimator block
the input of the block looks a bit strange, I guess gnuradio still
doesn't consume the appropriate number of input items...
decoding should work now

forgot to update the previously read sample

apparently, i can't really deduce the absolute bit values from
the "phase" of the chip sequence, however.

if the decoder can assume that every data packet starts with
a 1 (or 0) bit however, it should be able to decode the
sequence differentially
added example loopback flowgraph
decoding now seems to works consistently

by assuming that the first bit of a message is a 1 -- which
will always be the case when using the psk31 encoder to send data
(cf. cck_loopback.grc) -- the decoder is able to decode the chip
sequence