add InputMatrix#score api to calculate jaccard index by default
provide sorensen_coeffcient similarity measure

made available via the :measure option to input_matrix

provide sorensen index similarity measure

alternative similarity measures enabled by passing the `:measure`
option to the input_matrix macro, i.e.,

input_matrix :users, measure: :sorensen_index

rename sorensen_index -> sorensen_coefficient
specs for caculating scores with missing sets
extract Distance module

responsible for providing logic to calculate similarity distances
deprecate InputMatrix#calculate_jaccard
Merge pull request #4 from rossta/implement_input_matrix_score

Implement input matrix score