Adds configuration for progressive JPEG's

We've got a client system that doesn't effectively render progressive
jpeg images.  As such, we're looking to have a configuration value to
control the `progressive` flag on JPEG images.
Merge pull request #401 from SONIFI/master

Adds configuration for progressive JPEG's
WebM support
Merge pull request #402 from thumbor/webm

WebM support
Bump to 4.8.1
Add Statsd facilities for better instrumentation

Adds a stastd_client to the Context object to increase visibility
into Thumbor internals. For now, it is mostly insturmenting the
HTTP loader, and doing some logging in there.

I also added the HTTP_LOADER_CURL_ASYNC_HTTP_CLIENT config option
because the curl async client provides a lot more timing data
that is very useful to log.
Merge branch 'statsd_logging' into voxmedia_thumbor

Conflicts:
	thumbor/context.py
	thumbor/handlers/__init__.py
	thumbor/loaders/http_loader.py