Feature/test async and sync

[jitsedesmet]

This pull request aims to close Test both async and sync implementations #36. It also provides a way to say what kind of error should be thrown when calling testAllError this by writing 'error string'. If the type doesn't matter you can write ''. The pull request also provides a test that was previously asked for.

[coveralls]

Pull Request Test Coverage Report for Build 1094244852

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+6.3%) to 88.082%

---

Totals
Change from base Build 1093271228:	6.3%
Covered Lines:	1239
Relevant Lines:	1368

---

💛 - Coveralls