Added stcube methods.

Try:

```
library(spacetime)

# Import enviroCar track.
importEnviroCar = function(trackID, url = "https://envirocar.org/api/stable/tracks/") {
	require(RCurl)
	require(rgdal)
	require(rjson)
	url = getURL(paste(url, trackID, sep = ""))
	# Read data into spatial object.
	spdf = readOGR(dsn = url, layer = "OGRGeoJSON", verbose = FALSE)
	# Convert time from factor to POSIXct.
	time = as.POSIXct(spdf$time, format = "%Y-%m-%dT%H:%M:%SZ")
	# Convert phenomena from JSON to data frame.
	phenomena = lapply(as.character(spdf$phenomenons), fromJSON)
	values = lapply(phenomena, function(x) as.data.frame(lapply(x, function(y) y$value)))
	# Keep phenomena that were taken at each measurement point only.
	keeps = c()
	for(n in names(values[[1]])) {
		if(all(sapply(values, function(x) n %in% names(x))))
			keeps = c(keeps, n)
	}
	values = lapply(values, function(x) x[keeps])
	# Bind values together.
	data = do.call(rbind, values)
	units = as.data.frame(lapply(phenomena[[1]], function(x) x$unit))
	sp = SpatialPoints(coords = coordinates(spdf), proj4string = CRS("+proj=longlat"))
	stidf = STIDF(sp = sp, time = time, data = data)
	Track(track = stidf)
}

A1 = importEnviroCar("528cf1a3e4b0a727145df093")
A2 = importEnviroCar("528cf19ae4b0a727145deb40")
A3 = importEnviroCar("528cf194e4b0a727145de63d")

A = Tracks(tracks = list(A1, A2))
B = Tracks(tracks = list(A3))
Tr = TracksCollection(tracksCollection = list(A, B))

# Plot tracks in a space-time cube. As fetching the basemap takes its time,
# "showMap" defaults to FALSE. See ?stcube for detailed information.
stcube(A1, showMap = TRUE, col = "red")
stcube(B, showMap = FALSE)
stcube(Tr, showMap = TRUE)
```