Add writing-unit-tests-with-mockito

Add 2014-05-24-writing-unit-tests-with-mockito.md.
Fix several grammar errors in Mockito post

Fix several spelling and grammar errors in
2014-05-24-writing-unit-tests-with-mockito.md. Rewrite sentence
regarding JUnit assertion.
Merge remote-tracking branch 'origin/unit-tests-with-mockito'