serial_conn.py: use of parameter timeout

This is a change to fix the bug # 39. The Issue for that bug is incorrectly
described. The Parameter timeout is already in use, but only for reading
access. After that fix it is aso in use for writing access. The class
SerialConn inherits from Serial. The timeouts are regulated with the variables
_timeout for reading access and _writeTimeout for writing access.
See line 75-79 in serial_conn:

    self._writeTimeout = timeout
    self._timeout = timeout

See also:
Issue #39

Signed-off-by: Johannes Kroop <johkroop@gmail.com>
Acked-by: Erik Bernoth <bernoth@dresearch-fe.de>
Acked-by: Eik Binschek <binschek@dresearch-fe.de>
Acked-by: Steffen Sledz <sledz@dresearch-fe.de>
serial_conn.py: Added unit tests for timeouts.

Added 2 tests. One for the timeout, the other for the writeTimeout properties
used in serial_conn.SerialConn.read_until() and a mock for the pySerial API.

Getting the tests to work required changing the serial_conn as well. The
protected variables shouldn't be used, if not explicitely necessary. Instead
using the property is better and public API compliant.

Smaller cleanups:
 - changed the if, because a check for `txt == ""` is the same as a test for
   `not txt`.
 - if one local var is called `old_wto` it makes sense to call the other one
   `old_rto` as well

See Github Issue #39.

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>
Signed-off-by: Johannes Kroop <kroop@dresearch-fe.de>
Acked-by: Eik Binschek <binschek@dresearch-fe.de>
Acked-by: Steffen Sledz <sledz@dresearch-fe.de>