PHPUnit addition and subsequent changes

Unit testing shined light on several logic errors. Below is recap:

1: Constructor would have allowed empty arrays to be accepted for
processing

2: median function did not properly calculate median for Even length of
numbers

3: mode function did not properly calculate mode when more than one is
found

4: removed 80-character wrapping on exception for easier unit testing.
Merge pull request #3 from ethompson11/PHP

PHPUnit addition and subsequent changes