Merge lp://staging/~gz/charms/trusty/apache2-reverseproxy/gzip_encoded_extensions into lp://staging/~abentley/charms/trusty/apache2-reverseproxy/apache-website-interface

Adds a new configuration option to serve gzip encoded files with the correct content-encoding header

Two main things of interest:

* We should arguably set the content encoding in S3. This branch does not preclude doing that as well. Unfortunately s3cmd doesn't make it easy, and we'd have to fix up the previous files. See:

<https://github.com/s3tools/s3cmd/issues/396>

Which also raises the other side, if we have Content-Encoding set in S3, we really want the reverse proxy to *always* set accept gzip, and handle the vary/not vary stuff locally - rather than passing through to S3 and letting them decode and transfer uncompressed data.

* Rather than juse using mod_mime AddEncoding, this branch takes a more convoluted approach so it's possible to limit on multiple extensions. This should address Aaron's previous concern as we can then configure just ".log.gz" to be marked, we we know is safe to be displayed in browser.

In the LocationMatch block, we're first disabling mod_deflate (which otherwise futzes with things), then just setting the Content-Encoding header. No Vary business going on, we'd prefer that browsers and other tools get the encoded version.

It would be nice to add some unit tests, we can maybe refactor a bit later?