Improve check for Archive_Tar in the PKP PLN plugin

[mjordan]

We should improve how the PKP PLN plugin handles problems with the Archive_Tar extension (see this issue on the forum, for example) so that its absence from OJS's include path doesn't result in a WSOD on the plugin manager page. @asmecher can you advise on the best way to handle this? Tagging @ubermichael.

[mjordan]

We should also consider checking for other dependencies as per http://forum.pkp.sfu.ca/t/error-404-on-my-ojs/4764/16.

[asmecher]

Two possible approaches to avoiding breakage from missing dependencies:

• Check for the existence of the class before permitting the user to enable the plugin. (class_exists or similar)
• Only import the class just before it's required, not at the head of a file. In the case that the class is missing and the plugin is enabled, this will mean the whole site won't get broken (in the case of a generic plugin). Since generic plugins get loaded on every request, moving things into programatically-included code is a good thing, as it'll reduce unnecessary load on the system caused by generic plugins.

I had thought that this check was already in place, though?

[asmecher]

Looks good -- merged the PR.