Properly handle dependency checking

[JDGrimes]

This module will absolutely not work if WooCommerce isn't installed. We need to do two things.

• First, we need to determine which versions of WooCommerce we support.
• Then we need to decide how best to handle the situation when WooCommerce isn't installed.

For no. 2, I'm thinking that this might be something will will get up-ported into WordPoints itself eventually, if we design it to be versatile enough.

[JDGrimes]

It looks like WooCommerce only maintains the current version of plugin. For this reason, and due to the WordPoints standards, we'll be limiting support to the latest version of WooCommerce. We'll want to try to maintain as much compatibility as possible with the development version as well. So, at the moment we'll only support 2.2.x (current version is 2.2.10).

[JDGrimes]

It would be nice to tell the user about the module's requirements before they download it. However, that is beyond the scope of this module alone.

[JDGrimes]

I've created WordPoints/dependencies#1, but I don't think we'll wait for that. We can do something simple for now and revisit it later, maybe in 1.1.0