tokio-nursery: Abort tasks when NurseryStream is dropped

[jwodder]

Closes #30.

[codecov]

Codecov Report

Attention: Patch coverage is 91.11111% with 4 lines in your changes missing coverage. Please review.

Project coverage is 82.56%. Comparing base (04519bd) to head (335aea1).
Report is 37 commits behind head on master.

Files with missing lines	Patch %	Lines
crates/tokio-nursery/src/lib.rs	91.11%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
+ Coverage   82.41%   82.56%   +0.15%     
==========================================
  Files          32       32              
  Lines        1376     1411      +35     
==========================================
+ Hits         1134     1165      +31     
- Misses        242      246       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.