Setup property based test framework and add sample tests

[mkorszun]

Summary of changes

WHY

Introduce property based tests in order to:

• increase correctness of timex public api by discovering corner cases earlier
• reduce number of example based tests, which will not be able to cover all scenarios and edge cases anyway

What

• Use StreamData as a property based test framework
• Introduce a couple of sample property tests. We can not really add property tests for all the public functions anyway. It will be a longer process and we would like to start it with this PR.

Checklist

• New functions have typespecs, changed functions were updated
• Same for documentation, including moduledocs
• Tests were added or updated to cover changes
• Commits were squashed into a single coherent commit

[coveralls]

Coverage increased (+1.7%) to 72.502% when pulling 4068c84 on vorce:property_based_tests into 933588a on bitwalker:master.

[coveralls]

Coverage increased (+1.7%) to 72.502% when pulling 4068c84 on vorce:property_based_tests into 933588a on bitwalker:master.

[coveralls]

Coverage increased (+1.7%) to 72.502% when pulling 4068c84 on vorce:property_based_tests into 933588a on bitwalker:master.

[coveralls]

Coverage increased (+1.7%) to 72.502% when pulling 4068c84 on vorce:property_based_tests into 933588a on bitwalker:master.

[coveralls]

Coverage increased (+1.7%) to 72.502% when pulling 4068c84 on vorce:property_based_tests into 933588a on bitwalker:master.

[coveralls]

Coverage increased (+1.7%) to 72.502% when pulling 4068c84 on vorce:property_based_tests into 933588a on bitwalker:master.

[coveralls]

Coverage increased (+1.7%) to 72.502% when pulling 4068c84 on vorce:property_based_tests into 933588a on bitwalker:master.

[coveralls]

Coverage increased (+1.3%) to 73.09% when pulling e3576d8 on vorce:property_based_tests into 04dc72b on bitwalker:master.

[coveralls]

Coverage increased (+1.7%) to 72.502% when pulling 4068c84 on vorce:property_based_tests into 933588a on bitwalker:master.

[ckhrysze]

I may just be missing something, but why :tupple and not :tuple?

[mkorszun]

@ckhrysze typo :( - thanks for the feedback 👍