Encode special characters in planet's name.

Hi,

I was browsing randomly the catalogue website.
When you click on a planet which has a '+' in its name on the systems.html page, for instance on "2M 0746+20 b", it would display "System not found: 2M 0746 20 b".
See the table in: http://www.openexoplanetcatalogue.com/systems.html

You must encode the URL to ensure any special character is correctly url-encoded.
This commit makes the change and fix the broken links.