fix some typo. change var to val.
Refactor toString() by using Either.fold() to deconstruct the left and
right part of each Element instead of explicit pattern matching.
Refactor all conversion functions with Either.fold.
Merge branch 'master' into FizzBuzz
minor clean up with suggestions from ScalaStyle.
add toElement() so apply() can be a series of function compositions.
remove braces since now each function is one-liner.