Callback to allow ImageSource to be updated by WSGI middleware

Adds postprocess_image method to the base Server that allows WSGI
middleware to post process the ImageSource prior to it being returned in
the response. Commonly used for applying a graphic watermark or applying
a filter to the image.

The callback is associated with the key mapproxy.postprocess in the WSGI
environment. It is passed the ImageSource and is expected to return an
ImageSource.

The postprocess_image method is called from service/wms and service/tile
prior to the ImageSource being returned in a response.

Added unit tests for postprocess_image method.
Ensure that tiles that intersect a coverage are post processed
Rename callback and make decorate_img a regular method
Basic WMS system tests for decorate_img
Minor style fixes
Add test for tms / jpeg
Add more default parameters to callback and update tests
Pass environ and query_extend as a keyword parameters

Allows decorate_img callbacks to ignore them if not needed.
Add decorate_img support to wmts service
Documentation for decorate_img
Merge branch 'postprocess' of git://github.com/walkermatt/mapproxy into walkermatt-postprocess

Conflicts:
	doc/index.rst
add Matt Walker to AUTHORS
update decorate_img example