Refactor Loader Classes into LoaderBase

[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;

[dsparacio]

Update. The Retry API is already added in 2.0.