Option to prevent crawler / smaller fixes

[boerniee]

This pull request contains one new feature and some smaller fixes.

The new feature allows the user to add a meta tag to the page, preventing the photo-stream being indexed by crawlers. It can be turned on by setting the environment variable ALLOW_INDEXING to 0 and adds the meta tag named robots to the header section of the html document. This options defaults to 1, so crawlers are allowed.

Fixes:

• Added cdata content to some fields in the rss feed xml to have a valid xml.
• There was a link in the readme referring to the old repo. I have replaced it with a relative link to the current repo.
• The meta url tag contained the url detected by the application. That url was wrong when using a reverse proxy. Now it uses the environment configuration url.

[waschinski]

Bernhard, thanks for the new feature as well as fixing some general problems!