Implement timeout on acquiring connection from pool

[Pliner]

What do these changes do?

PR adds timeout for the acquiring a connection from the pool.

Are there changes in behavior for the user?

Pool.acquire will raise TimeoutError if no connection is returned from the pool in specified timeout.

Related issue number

Fixes #130, #218

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

[codecov]

Codecov Report

Merging #766 (03be802) into master (b5d3490) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #766   +/-   ##
=======================================
  Coverage   92.89%   92.89%           
=======================================
  Files          13       13           
  Lines        1562     1563    +1     
  Branches      179      179           
=======================================
+ Hits         1451     1452    +1     
  Misses         79       79           
  Partials       32       32           

Impacted Files	Coverage Δ
aiopg/__init__.py	100.00% <100.00%> (ø)
aiopg/pool.py	100.00% <100.00%> (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 b5d3490...03be802. Read the comment docs.