Merge branch 'subscribe-idn-feed' into 'master'

Merge branch 'subscribe-idn-feed' into 'master'

Subscribe to feed with Internationalized Domain Name

Currently you cannot subscribe to feeds on hosts with internationalized domain names (IDNA) within tt-rss. You need to manually convert them to punycode to subscribe to them.

This patch adds code to detect IDNA and convert them to punycode in fix_url() if possible on the system. This requires PHP IDN functions (e.g. on Debian Jessie this needs php5-intl to be installed), so a notice is added to the installer sanity check.

See merge request !37