Revamp MediaHelper

[ndm2]

As described in https://github.com/bmcclure/CakePHP-Media-Plugin/issues/49#issuecomment-28238733, this patch switches back to extending AppHelper, and ditches manual tag generation and overriding Helper::_parseAttributes() in favor of using HtmlHelper::useTag() and utilizing config files (as suggested by @gBokiau in https://github.com/bmcclure/CakePHP-Media-Plugin/pull/50#issuecomment-28200398) to configure HtmlHelper with proper tags and minimizable attributes.

It should be backwards compatible and resolve #50.

[bmcclure]

I like this approach. It's fairly substantial, but it seems relevant to the issue at hand, and @ndm2 has convinced me that going back to extending AppHelper is the cleanest way to go.

@ndm2, based on the last two comments are you planning to add another commit to this PR to load both configs if a custom one is specified? I have not tested these changes, but after looking over the code I do not see any other issues with this.

And finally, just to confirm, @gBokiau, are you in agreement that this is a good approach, since I believe it would prevent merging https://github.com/bmcclure/CakePHP-Media-Plugin/pull/50? Are there any changes/fixes in your existing PR that aren't covered by this one?

[ndm2]

Yes, I'm going to add another commit that adds support for loading both configurations, just wanted to wait until you had a look at the whole thing too.

[bmcclure]

With the latest commit added, I don't see any further issues that would keep me from merging this in.

I think to the extent that we can keep backwards compatibility without a lot of extra work/code, we should. Down the line if features or changes are introduced that make the plugin no longer compatible all the way back to 2.0.x (for a good enough reason, of course), we could revisit cleaning it up a bit and removing some things that would no longer be necessary.

[ndm2]

Sounds good to me.