Added API framework and a package stats endpoint

[ColdenCullen]

The idea is that this should allow us to start adding badges to shields.io.

This PR also leads the way towards adding more stats in the future.

[PxlBuzzard]

If you switched the import in registry.d#130 to be days instead of weeks, you could do the same math for perWeek and perMonth and add a perDay as well.

[ColdenCullen]

@PxlBuzzard done.

[s-ludwig]

My only nitpick would be to use a statically typed struct instead of Json for the return type of getPackageStats. I'd also exclude any downloads that are not for tagged versions (i.e. eclude those that have a version field starting with ~), because those are re-downloaded for every dub upgrade and thus would potentially yield unrealistic numbers.

[s-ludwig]

(Maybe using Nullable to be able to express the Json(null))

[ColdenCullen]

Done!

As far as the nullable goes, I realized that the db will throw if the package isn't found, so it's trivial to just pass the exception up another level.

[ColdenCullen]

Ping?

[ColdenCullen]

Ping?

[ColdenCullen]

Ping @s-ludwig again? I really want dub shields!

[MartinNowak]

How about daily, weekly and monthly instead?

[ColdenCullen]

Works for me.

[MartinNowak]

What about querying downloads by version?

[ColdenCullen]

@MartinNowak I think I've addressed all of your concerns at this point, let me know if you spot anything else that could be better.

[ColdenCullen]

@MartinNowak now that your changes have been merged, is there anything else we're waiting on?

[ColdenCullen]

Ping?

[MartinNowak]

thx

[MartinNowak]

Shield now available on http://shields.io/.