Add unit tests and sensible coverage

[sadikovi]

This PR adds some unit-tests around parsing different events into JSON and abnormal behaviour of socket or corrupted events.

Tests can be run using sbt test.
Tests with coverage can be run using sbt coverage test coverageReport.
I am not sure if there is a CI for repo, but it would be good to include tests.

Here is the summary for this patch:

[info] Total number of tests run: 17
[info] Suites: completed 1, aborted 0
[info] Tests: succeeded 17, failed 0, canceled 0, ignored 0, pending 0
[info] All tests passed.
...
[info] Statement coverage.: 89.62%
[info] Branch coverage....: 80.00%
[info] Coverage reports completed
[info] Coverage is above minimum [89.62% > 80.0%]
[info] All done. Coverage was [89.62%]

Though It is not great coverage and tests, I hope checks will help with migrating to different Spark versions.

[sadikovi]

@ryan-williams could you have a look at this PR? Thanks!
Would be good to get some feedback on usefulness of unit-tests.

[sadikovi]

Closes #11.