Made some changes to species details and curl options

hey @Ironholds 

Feel free to reject these changes. Basically, I 

* Changed _wikispecies_ to _species_ in the url construction as _wikispecies.wikimedia.org_ doesn't resolve, but _species.wikimedia.org_ does
* The `rvcontentformat` parameter in the API call was causing the species call to fail, though seemed to work for others. For example, this call http://species.wikimedia.org/w/api.php?format=json&rvcontentformat=text/css&action=query&prop=revisions&rvdir=older&rvprop=content&titles=Helianthus gives 

```
{
   servedby: "mw1148",
   error: {
      code: "rvbadformat",
      info: "The requested format text/css is not supported for content model wikitext used by Helianthus"
   }
}
```

So I removed that parameter for the species calls. 

* I added as an example a parameter `curlopts` to the `wiki_page` and `wiki_pagecats` functions to let users pass in curl options to the `httr::GET` call. So they can do things like

```coffee
conn <- wiki_con("english", "species")
library("httr")
```

verbose, print curl info

```coffee
wiki_page(con = conn, pages = "Helianthus", "timestamp", curlopts = verbose())
```

make call stop if taking longer than X seconds

```coffee
wiki_page(con = conn, pages = "Helianthus", "timestamp", curlopts = timeout(seconds = 0.1))
```