Exporting and importing selfoss OPML files (#103)

This extends the OPML controller to add exporting of all sources as an OPML file from the sources page.

From the exported OPML file, all sources can be re-imported to selfoss with the correct spout, settings and tag colors and *almost* all sources can be imported to other RSS readers.

This required one addition to the spout interface: The `spout::getXmlUrl(array $params)` function.
This function receives the same parameter array as the spout::load function and should return either a single URL as `string` (the RSS feed used by the source) or `false` (e.g. if the source uses an API instead of an RSS feed).
So, if a spout wants to be importable to other RSS readers, it has to implement this function.

All selfoss-specific information in the exported OPML is in a separate namespace, so the OPML should be compatible with most other RSS readers.

So far, I've tested it on PHP 5.4.14 and importing from/to Google Reader and Netvibes, but everyone should give it a try to see if importing/exporting works correctly.