added context support

[cmoad]

Added support for passing context into all calls that hit the database. I was unable to get the test suite running on my machine to verify. Opened #63 related to that problem.

[cmoad]

Resolves #59

[cmoad]

Pushed a fix for #63. I'll work on adding more coverage for context tests now.

[codecov]

Codecov Report

Merging #64 into master will decrease coverage by 2.26%.
The diff coverage is 29.21%.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
- Coverage   82.98%   80.72%   -2.27%     
==========================================
  Files          17       17              
  Lines        2087     2169      +82     
==========================================
+ Hits         1732     1751      +19     
- Misses        247      310      +63     
  Partials      108      108

Impacted Files	Coverage Δ
dataset_actions.go	52.38% <0%> (-32.24%)	⬇️
crud_exec.go	86.95% <100%> (+0.66%)	⬆️
database.go	69.65% <18.75%> (-25.2%)	⬇️

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[cmoad]

We've been using this code for a while with success. The checks are failing for go version 1.6 because context wasn't a standard library. It's failing with go 1.7 because the sql package didn't have context support. This branch works in go 1.8 and up. There's nothing I can do to fix those tests. Perhaps this PR should be included in a new major version that only supports go 1.8 and up?

[doug-martin]

Yeah, I think that would be the best route. If you want to update travis to test 1.8, 1.9, and latest` that would be great!