Improve CPU performance by using json-iterator instead of encoding/json

[debnath]

Hi, not sure if you wanted to add in vendor'd dependencies into your package, but figured I'll open a PR and see if you wanted the change.

I noticed while running pprof on my production application that the json Unmarshalling of package nvellon/hal was in the top5 CPU consuming functions. So I swapped out encoding/json with json-iterator on my fork and noticed an immediate improvement in both CPU consumption as well as latency.

I also noticed the unit tests fail in go1.12, so I made some minor changes to the tests.

[coveralls]

Coverage remained the same at 86.503% when pulling dbf01ee on debnath:master into d5526db on nvellon:master.

[debnath]

Looks like the travisci check has failed due to not being to import in package context in go1.4. context was not included until go1.9, so this makes sense.

Please go ahead and delete this PR if you wanted to retain go1.4 compatibility.