support to configure different redis-server db by adding REDIS_DB

[featureoverload]

No description provided.

[codecov]

Codecov Report

Merging #166 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #166   +/-   ##
=======================================
  Coverage   92.85%   92.85%           
=======================================
  Files          18       18           
  Lines         728      728           
  Branches       57       57           
=======================================
  Hits          676      676           
  Misses         36       36           
  Partials       16       16           

Impacted Files	Coverage Δ
src/scrapy_redis/connection.py	100.00% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 40440c8...171f72d. Read the comment docs.

[botzill]

This seems OK for me, @rmax can we merge this?