This commit adds the command line options "-o" and "--logfile" that
enable logging to file.  This functionality required minor restructuring
that led to a simplification of the logging code.  In particular:

- We get rid of log.py because it was never really necessary.

- We use named loggers, so it's easier to figure out what module logged
  a given message.

- We rename our logging object from "logger" to "log" to make the code
  more readable.

- The restructuring means that we now also get to see Stem's logging
  messages.

This command line option hopefully makes it a little bit more difficult
for vigilant adversaries to discover our scanning activity.  It adds a
random value to the time delay between two subsequent circuit creations.
As a result, if an adversary monitors circuit creation timings, our
signal should be less periodic and thus less obvious.

We implement this feature by exposing a command line argument that
determines a random time delay in the interval [0, DELAY_NOISE).  This
delay is then either added to or subtracted from (both with probability
0.5) the given build delay.  By default, the argument is 0, and
therefore not in effect.

Travis CI discovered that PyPy in version 2.6.1 apparently lacks the
ENOTSUP variable.  This commit defines the variable if it does not exist
already.  The exact test error was:

  ============================= test session starts ==============================

  platform linux2 -- Python 2.7.10[pypy-2.6.1-final], pytest-2.8.5, py-1.4.31, pluggy-0.3.1

  rootdir: /home/travis/build/NullHypothesis/exitmap, inifile:

  plugins: cov-2.2.1

  collected 8 items / 1 errors

  test/test_relayselector.py .

  test/test_stats.py .

  test/test_util.py ......

  ==================================== ERRORS ====================================

  ____________________ ERROR collecting test/test_torsocks.py ____________________

  test/test_torsocks.py:24: in <module>

      import torsocks

  src/torsocks.py:61: in <module>

      0x07: errno.ENOTSUP,      # Command not supported / protocol error

  E   AttributeError: 'module' object has no attribute 'ENOTSUP'

After torsocks.py and relayselector.py have been refactored, we will
have to rewrite the unit tests.

RFC: New module 'rtt' and a bunch of changes in aid of what it does