Refactor Loader Classes into LoaderBase

Created by: dsparacio

There are too many shared aspects of loader classes for manifest, xlink, fragment. Lets refactor and also think of a way to abstract it a bit so we can have XHR loading or p2p or streams, multicast etc.

Also want to be able to set values for these via API const RETRY_ATTEMPTS = X; const RETRY_INTERVAL = X;