Separating singular handler type to ease reuse/chaining

After much usage, turns out that having a single handler type that processes both channel and item handlers is not very useful. I've found that there is great utility in create handler chains e.g. urlFilterItemHandler is chained to a MemcacheItemHandler etc. However, with the enforcement of a singular handler to process both chains, this led to either "empty" placeholders for some handlers or even worse, duplication of effort.

It is much more useful, and much more open to reuse, to have the handlers be separate chains that can be configured any way the user wants. To this end I have stripped out all the singular handler type stuff and implemented the remainder as separate handler chains. This has the added benefit of removing the rather nasty HandlerBonder type.

In addition to this I have ensured that the New() feed function still operates exactly as previous versions of the package, so this is still backwards compatible.

This should improve things and make things much simpler/reusable/chainable.

Cheers!
Ben