fix stentura high CPU usage

This patch is a fix for high CPU usage in stentura.py.  It sets a small default timeout on the serial port (0.05 seconds) for reading data.  That timeout becomes the granularity for interrupting the process of reading data in _read_data().  _read_data() attempts to read as much data as it can within that small timeout period.  I moved the check for the overall timeout up to _read_packet().

Verified it functions with low CPU usage on Ubuntu 13.04.