roman-numerals: Adopt functional style

As discussed in #79.
beer-song: Adopt functional style

As discussed in #79.

The original "Beer.sing" method is replaced by a "song" function -
"song" being on the same level of abstraction as "verse".
anagram: Adopt functional style

As discussed in #79.

This also adds a test case to make sure that differently cased
versions of a word aren't recognized as anagrams. E.g. "the"
and "The" aren't anagrams.
Merge pull request #93 from sjakobi/functional-roman-numerals

roman-numerals: Adopt functional style
Merge pull request #84 from sjakobi/functional-beer-song

beer-song: Adopt functional style
Merge pull request #83 from sjakobi/functional-anagram

anagram: Adopt functional style