Switches to BigDecimal

Closes alown/eMonies#19
Ensures all people have entries in SparseArray

Closes alown/eMonies#21
Adds purchase ID to table and detail pages

Closes #18
Provides UI for deleting purchases

Closes #14
Merge branch 'feature/bigdecimal'
Merge branch 'feature/delete-purchases'
Merge branch 'feature/show-purchase-id'