Add additional tests that initial "xr" is treated as a vowel sound as is initial "y\<consonant\>".  tests were also added to assure that adding handling of these special cases does not break appropriate handling of other initial "x" and initial "y" cases.

An example exercise solution that passes these new tests is substituted for the original example code.
Merge pull request #1054 from dpneumo/Additional-PigLatin-tests

Add additional tests to pig-latin that initial "xr" is treated as a vowel sound as is...