Improved S3 summary

In response to @mkoo's question yesterday, I improved the print method to clearly show how many pages are being retrieved and also differentiate what's downloaded on a call as opposed to what's available on the server. With this update, the `ee_observation` calls look like this:

```coffee
> aves <- ee_observations(clss = "aves")
Search contains 171117 observations (downloading 1 of 6845 pages)
> aves
Total results on the server: 171117 
Args: 
country = United States 
clss = aves 
georeferenced = FALSE 
page_size = 25 
page = 1 
Type: observations 
Number of results retrieved: 25 
```

```coffee
> aves <- ee_observations(clss = "aves", page = 1:2)
Search contains 171117 observations (downloading 2 of 6845 pages)
> aves
Total results on the server: 171117 
Args: 
country = United States 
clss = aves 
georeferenced = FALSE 
page_size = 25 
page = 1 2 
Type: observations 
Number of results retrieved: 50 
```

With other types of search (e.g. photos)

```coffee
> z <- ee_photos()
Search contains 43708 photos (downloading 1 of 1 pages )

> z
Total results on the server: 43708 
Args: 
page_size = 25 
georeferenced = 0 
page = 1 
Type: photos 
Number of results retrieved: 25 

```