compensate for omxplayer's inaccurate stats output

Currently the stats output of omxplayer is not 100% accurate. It is
ahead of the video by around 100ms in the latest omxplayer build.
compensate for IO overhead (subprocess and UDP stats reading)

Currently omxplayer prints its stats over 400 times a second
to the console. Thus the pexpect call in read_position_local will be
tirggered equally often and equal amount of packages is sent over
the wire. As a result, position_local will be slightly behind.
The slave has slightly larger overhead because it additionally receives
network traffic in a seperate thread.
introduce SKIP_SIZE, for easy setup of custom omxplayer builds
wait_for_sync review
update README
get rid of global 0.05 s sleep