Add string buffer data structure.

String buffer reduces runtime of concatenating strings.
Creates buffer array to store strings while concatenating.
Once all strings to be joined have been added to the buffer
the program concatenates the characters together once.

Alternatively the program might concatenate the strings for
each word in the array of strings to be joined.

Monkey patched String class to add class method called join_words.

.join_words creates a StringBuffer object and appends all words to
the buffer. Once all words have been appended it call #to_s on
the buffer which concatenates the strings.