Update to support gzip in same way as DropWizard (#38)

This adds a number of new configuration settings, with the leaf keys matching the ones used by DropWizard. The new configuration settings are documented in the README.

In particular, this adds support for decompressing gzip-encoded request bodies (by way of the BiDiGzipFilter). The default mime type handling has changed; instead of defaulting to only compressing a particular list of mime types, the new default behavior is to compress all non-gzip mime types (matching the default behavior of Jetty's GzipFilter).

The old list was:
"application/json", "application/xml", "text/html", "text/plain", "application/javascript",
"application/x-javascript", "text/javascript", "text/css", "text/xml", "image/svg+xml"
I considered that.  In general, my approach was to use the Dropwizard defaults when applicable.  In this specific case, I think that using the Dropwizard default (which is also the Jetty default; compress all types unless blacklisted or a list is explicitly specified) is the better approach.  Maintaining our own list of default compressed mime types is something that would be likely to change over time (potentially breaking compatibility with some apps each time), and particular mime types not being on the default list is likely to be surprising to users.  It's much easier to convey "we compress everything by default; if you don't want that, specify a whitelist or a blacklist", rather than "we compress certain types by default; check if the types you want are already in the list, and if not, overwrite it with a different list... or overwrite it with an empty list to compress everything".