Only add the features component if the corresponding uuid module is enabled

I have a site with > 10,000 nodes that I was trying to export taxonomy terms from. Every time I went to create a feature, I got a timeout error because it was trying to load all 10,000 nodes to create uuid_node entries for them. I thought I could get around this by disabling the uuid_node module, but it didn't work.

This patch checks to make sure that uuid_node and uuid_taxonomy are enabled before adding their respective feature callbacks.