Add caching layer for skins

Added basic in-memory caching of avatars, which is quite fast and would help out a bit for serving the most viewed avatars. Set a rough limit of 64 MB of in-memory cached data.

```
Looking at response times, running Siege on a Macbook Air.

============================================================
Response time before caching was added:

For all requests: 0.082 secs

============================================================
Response time after caching was added:

Getting the same avatar 100 times: 0.014 secs # best case, >4x increase in speed!
Getting a unique avatar 100 times: 0.083 secs # worse case -, not much different from before caching
Getting the same set of 10 avatars 1000 times: 0.013 secs

============================================================
Letting it run and taking a nap, peak memory usage in caching. Expected
it to peak around 64 MB. Looks like there's actually a memory issue elsewhere.
Increased ad infinitum, even on the current master branch without my caching addition.

============================================================
Am now banned from Mojang API: definitely.
```

The timings in the request log makes me much more happy than before :)

```
[15:25:36.474659] INFO Serving skin for connor4312 (184+0+6=191ms) md5: d05a3eda6f8b1d3b8dfae6315c3aae65
[15:25:37.165298] INFO Serving skin for connor4312 (0+0+5=5ms) md5: d05a3eda6f8b1d3b8dfae6315c3aae65
[15:25:37.442426] INFO Serving skin for connor4312 (0+0+4=4ms) md5: d05a3eda6f8b1d3b8dfae6315c3aae65
[15:25:37.710710] INFO Serving skin for connor4312 (0+0+4=4ms) md5: d05a3eda6f8b1d3b8dfae6315c3aae65
[15:25:37.979408] INFO Serving skin for connor4312 (0+0+4=4ms) md5: d05a3eda6f8b1d3b8dfae6315c3aae65
```
The issue though is (for me at least/other people who use imgd for their own use), unless you are restarting your service every few days, one could end up with a fairly stale cache (I doubt I would fill it my usage)? 

Or, for minotar, if you have an incredibly popular skin (eg. located in a prominent place), and it's always being bumped to the top of the list/never being trimmed off the bottom, that will end up potentially very stale?