Clean up getphotos function...

* Fail immediately if HTTP request fails (gives better error message)
* Use numeric for loop instead of ipairs iterator
* Rename "items" to "entries"
Clean up getalbums function...

* Disable photo fetching by default -- enabled by passing a size parameter
* Wrap HTTP request with assert(), preventing late, cryptic error messages
Fetch photos by username/albumid instead of passing a URL