Added link to releases on Github

Also changed links to other webs to open in new windows.
Changed default map center from Prague 8 to Prague
Improved detection geocoder failure and used random point in district area when geocoder fails

Failure to geocode was not always detected correctly. Also default map center was used on geocoder failure which was not right if place was related to a district.
Added robots.txt and sitemap.xml

Relates to issue #25.
Added favicon.ico

Relates to issue #25.
Added gzip support

Relates to issue #25.
Extended meta information

Relates to issue #25.
Minified css files into one

Relates to issue #25.