make read() fail with TwistedConnector
VRTClient with callback for packets arriving
vrt_callback: pass through from TwistedConnector -> VRTClientFactory -> VRTClient
update gui for TwistedConnector changes
remove twisted_util (no longer used)
update twisted_show_i_q example
Merge pull request #15 from wardi/vrt-callback

TwistedConnector: use simpler, more general callback approach for when VRT data arrivals
remove outdated twisted test
removed old plotting mechanism, added pyqtgraph to plot, added max hold and grid hotkeys
added trigger controls
WSA4000: stream_start, stream_stop, stream_status
:sweep:flush no longer exists
WSA4000: add system_flush, system_abort
Revert ":sweep:flush no longer exists"

This reverts commit 9657756f253b71286085b664df06928bf496e6f3.
:sweep:flush is dead; long live :system:flush
rename system_abort to abort, more like other methods
WSA4000: improve spp()
stream speed test example
wsa4000gui: start using streaming mode
wsa4000gui: keep all vrt context values received