Updated TokenChain.java; added method length() which returns the length
of the token chain, added method get() which returns the Token element
at some specific index in the chain, added method getSubChain() which
returns a TokenChain representing a portion of the origin TokenChain,
added method add(TokenChain chain) that merges together another
TokenChain. Updated TokenScanner.java; added method getTokensBetween()
which returs the tokens in a TokenChain that lie between a given token
delimiter.