Use a monotonic clock instead of time.time()

[eldipa]

Describe the feature you'd like
time.time() is sensible to time shifts due to clock readjustments via NTP
or others.

This may lead to unexpected timeouts, event to absurd elapsed time
notifications (like saying that an example took hours)

The new Python 3.x's monotonic clock should be immune to these.

The codes that use time.time() to calculate delta of times are:

• byexample/byexample/modules/progress.py

  Line 67 in 3389113

  self.begin = time.time()

• byexample/byexample/common.py

  Line 395 in 3389113

  return time.time()

• byexample/byexample/expected.py

  Line 503 in 3389113

  begin = time.time()