rna-transcription: Use functional solution

See #79 for a discussion.
word-count: Adopt functional style

As discussed in #79.
nucleotide-counts: Adopt functional style

As discussed in #79.

Two test cases that don't translate to a functional solution are
removed.
leap: Adopt functional style

As discussed in #79.
gigasecond: Adopt functional style

As discussed in #79.
point-mutations: Adopt functional style

As discussed in #79.

This also removes the last test case which was specificically
conceived for a class-based solution but didn't actually work
in the first place.
series: Adopt functional style

As discussed in #79.
largest-series-product: Adopt functional style

As discussed in #79.

This also adds a note that suggests that the code for the series
exercise be reused.
octal: Adopt functional style

As discussed in #79.

The original Octal.to_decimal method is replaced by the parse_octal
function which seems more aptly named.
scrabble-score: Adopt functional style

As discussed in #79.
Merge pull request #81 from sjakobi/functional-rna-transcription

rna-transcription: Use functional solution
Merge pull request #82 from sjakobi/functional-word-count

word-count: Adopt functional style
Merge pull request #85 from sjakobi/functional-nucleotide-counts

nucleotide-counts: Adopt functional style
Merge pull request #86 from sjakobi/functional-point-mutations

point-mutations: Adopt functional style
Merge pull request #89 from sjakobi/functional-series

series: Adopt functional style
Merge pull request #88 from sjakobi/functional-gigasecond

gigasecond: Adopt functional style
Merge pull request #90 from sjakobi/functional-largest-series-product

largest-series-product: Adopt functional style
Merge pull request #91 from sjakobi/functional-octal

octal: Adopt functional style
Merge pull request #92 from sjakobi/functional-scrabble-score

scrabble-score: Adopt functional style
Merge pull request #87 from sjakobi/functional-leap

leap: Adopt functional style