avoid to format code in javadoc as it creates too much space before and after the code
Fixes #264 : Skip unexpected elements when there are none

This applies to the following methods :
- containsOnly
- containsOnlyKeys
- containsExactly
Use 2 spaces instead of one in error message when displaying a value on a new line.
Remove isEmpty in BasicErrorMessageFactory in favor or Iterables#isNullOrEmpty.
Reformat error messages to be more readable.
Use assertj assertions instead of JUnit#assertEquals