Implement task group associated to a Context & more

[alitto]

• Upgrade go version to 1.8
• Implement new method in WorkerPool to create a group of tasks associated to a context GroupContext(ctx context.Context)
• Move TaskGroup to a separate file
• Move tests related to task groups to a separate file

[codecov-commenter]

Codecov Report

Merging #30 (f9197db) into master (d4c09d4) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #30   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         3    +1     
  Lines          329       373   +44     
=========================================
+ Hits           329       373   +44     

Impacted Files	Coverage Δ
group.go	100.00% <100.00%> (ø)
pond.go	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 d4c09d4...f9197db. Read the comment docs.