Remove a duplicate word

[MRG+1] Favoring web scraping over screen scraping in the descriptions

[MRG+1] Python logging

install service_identity package in tests to prevent warnings

Relax SpiderLoader interface check

There are two functions, `configure_logging` and `log_scrapy_info` which
intend to replace scrapy.log.start and scrapy.log.scrapy_info
respectively.

Creating new functions makes evident the backward incompatible change of
using another logging system, and since the Python logging module is a
standard builtin, additional helpers make sense to be on a scrapy/utils
file.

This allows to remove `get_testlog` helper, `flushLoggedErrors` from
twisted.trial.unittest.TestCase and Twisted log observers created for
each test on conftest.py.

Changes:
 - Each module takes 'scrapy' logger and logs through it
 - Lazy string evaluation in all log messages
 - Added missing log messages in scrapy/core/engine.py
 - Contextual data such as crawler or spider instances, and failures