Extend spout classes with a getXmlUrl function

This adds a new function to the spout interface and its subclasses:
spout::getXmlUrl(array $params)

getXmlUrl receives the same $params array as the spout::load function.

getXmlUrl can use the $params argument to determine the URL of the RSS
feed used by the source and should return a single URL as a string.
If the spout does not fetch its content from an RSS feed internally,
(e.g. because it uses an API instead), getXmlUrl should return false.
Add OPML exporting and adapt OPML controller

This rewrites large parts of the OPML controller and adds exporting
of all subscriptions as OPML files via http://selfoss/opmlexport.

The generated OPML files should be compliant with the OPML 2.0 specs
(http://dev.opml.org/spec2.html). The subscription list is written as
a tree, with each subscription being a child of each of its tags.

Attributes and elements that aren't part of the OPML 2.0 specs are
saved in a seperate namespace. These are: the tag colors, spout names,
spout parameters and other meta information.

OPML importing has been refactored and adapted to read the information
from the selfoss namespace and to import tag colors and spouts.
Integrate OPML export in UI, update translations
Merge pull request #305 from seanrand/new/opml-export

Exporting and importing selfoss OPML files (#103)