Add dependency on rhumsaa/array_column (polyfill for PHP < 5.5)
(A dependency on array_column was introduced in 9ac2288c)
Reorganize the way home and away teams are represented in the object model
(a single OneToMany indexed, bidirectional association rather than two OneToOne associations)
Closes #21
Update the CompetitionService tests to match the implementation
(which was changed in commit 9ac2288 for #15)