Add global download statistics

[wsnipex]

This adds a new handler in http.go that uses existing file download statistics to show a global download statistics page.
We have this running on our server for a while: http://mirrors.kodi.tv/stats?downloadstats

• downloadstats param takes the same optional date format as ?stats (period)
• simple filename filter(no wildcards atm): &filter=example
• limit parameter that limits results returned: &limit=[int]
  default=100
  set to 0 to disable
• optional json output: &format=json

examples:
http://mirrors.kodi.tv/stats?downloadstats=2015-08-17 <- all DLs from yesterday(17th August)
http://mirrors.kodi.tv/stats?downloadsta...oviedb.org <- all DLs from Juli containing metadata.themoviedb.org in the file path

The template file could use some web dev hands on, its currently very basic.

[etix]

I've started to merge your code on top of the refactored master but few some modifications though. I really like the idea of specifying a path in the configuration file instead of relying on query arguments so I removed it from your pull request and will implement it globally at a later time.