v0.4.0: fix 0.7 deprecations and drop 0.5 (#40)

* fix 0.7 deprecation warnings

* remove mutability on some wrappers

v0.3.0

Fix deprecation and drop 0.4 support.

v0.2.0

Remove the old API in favor of the one introduced in v0.1.

v0.1.1

Update to work with Julia 0.5.

v0.1.0

Implements a new API for working with contour levels.

c = contours(xs, ys, zs) # a contour collection with levels selected automatically
for lvl in levels(c)
    l = level(lvl) # the z-value of the current contour level
    for line in lines(lvl) # each contour level might consist of multiple, disjoint paths
       x, y = coordinates(line) # coordinates for the line segment of the current path
       # hypothetical example: plot(x, y; color = l)
    end
end