Add equality comparison method to dhist

[martintoreilly]

R does not know how to compare two dhists. Implement == for this class.

Implementation

Noted here as it took quite a bit of Google-foo to figure out how to name methods that are operators (the backtick magic)

`==.className` <- function(x, y) {
    all(mapply(`==`, x, y)
}

[martintoreilly]

Closed by PR #80