Cache download URL

[qgustavor]

Calling .loadAttributes() on files loads the download URL from the API but it's not stored so a second call get's required then .download() is called. Caching would help in case .download() is called many times, like to download multiple parts from a file (but not the entire file).

The only catch I see on doing that is handling when the download URL expires. From my experience seems it expires automatically when the end of the file is downloaded and, probably, when it's not acessed after some time.

We need to handle expiration (checking the HTTP response codes and then requesting a new download URL if needed) and test how MEGA behaves to avoid an additional request in situations where we know it already expired.

[qgustavor]

Closing in favor of this: #89