Log versions information at startup

[redapple]

This is an example startup log with this patch:

$ scrapy shell
2017-07-27 17:22:09 [scrapy.utils.log] INFO: Scrapy 1.4.0 started (bot: scrapybot)
2017-07-27 17:22:09 [scrapy.utils.log] INFO: Versions: lxml 3.7.3.0, libxml2 2.9.3, cssselect 1.0.1, parsel 1.2.0, w3lib 1.17.0, Twisted 17.1.0, Python 2.7.12+ (default, Sep 17 2016, 12:08:02) - [GCC 6.2.0 20160914], pyOpenSSL 17.0.0 (OpenSSL 1.0.2g  1 Mar 2016), Platform Linux-4.8.0-59-generic-x86_64-with-Ubuntu-16.10-yakkety}
2017-07-27 17:22:09 [scrapy.utils.log] INFO: Overridden settings: {'LOGSTATS_INTERVAL': 0, 'DUPEFILTER_CLASS': 'scrapy.dupefilters.BaseDupeFilter'}

@kmike , were you thinking of something like this?

[codecov-io]

Codecov Report

Merging #2857 into master will decrease coverage by <.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #2857      +/-   ##
==========================================
- Coverage    84.7%   84.69%   -0.01%     
==========================================
  Files         163      164       +1     
  Lines        9191     9189       -2     
  Branches     1367     1369       +2     
==========================================
- Hits         7785     7783       -2     
  Misses       1154     1154              
  Partials      252      252

Impacted Files	Coverage Δ
scrapy/commands/version.py	100% <100%> (+8.88%)	⬆️
scrapy/utils/log.py	88.76% <100%> (+0.25%)	⬆️
scrapy/utils/versions.py	82.6% <82.6%> (ø)

[kmike]

Looks awesome!

What do you think about creating something like scrapy.utils.versions instead of putting it to utils.log?

[kmike]

Thanks @redapple! This PR would make it easier to debug issues.