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)
* update README.md
* hide opml things on smartphone
* style improvements
Update tr.ini
Sanitize tag list and title when adding/editing a source
Merge pull request #307 from osmank3/patch-1

Update tr.ini
Merge pull request #308 from seanrand/fix/source-whitespaces

Sanitize tag list and title when adding/editing a source
Update ja.ini (lang_source_export)
Merge pull request #310 from crckyl/lang_ja

Update ja.ini (lang_source_export)
Update ru.ini

Export translation
Integrate OPML export in UI, update translations
Merge pull request #312 from Kleofass/patch-1

Integrate OPML export in UI, update translations
Merge pull request #311 from glesik/patch-1

Update ru.ini
new norwegian translation (thanks a lot to Kjetil Elde)
set default title if no one was given by the feed
* fix typo
* improve ContentLoader.php
cleanup ContentLoader.php
Added FR translation for export sources

Added translation for export sources