Improve checking requirements

During install, the REQUIREMENT_INFO severity of the two feeds requirements is a bit irritating, especially if the requirements are met and it should be REQUIREMENT_OK.
We can't be sure about SimplePie (because libraries module is not yet enabled) but we can at least try, because the installation profile (or the admin) might have rightaway installed the library.
In any case, SimpleXML shouldn't throw INFO severity if present.

In runtime, we should be a bit clearer about the status and what to do about it. For SimplePie, we should speak of 'Installed' vs. 'Not installed' instead of 'Enabled' vs. 'Not enabled', because this is not merely a matter of configuration.
Also, we should give some more help what version to download, where to put the file and - yes, unfortunately also how to rename it.
Finally, there is a little typo ("seek instructions from the server mantainers").
Also refactored the code a bit to make it a bit clearer.