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
mapproxy-util now supports WMS Version 1.3.0; updated docs and tests
refactored code to match external code example
Rename callback and make decorate_img a regular method
Basic WMS system tests for decorate_img
added explicit version option
fixed cmd line args for tests
capabilities printer now works with sublayers;
in addition to the layers, metadata is also parsed
Merge branch 'master' of git://github.com/mapproxy/mapproxy into mp-util_130capabilities
updated doc to reflect the changes made to wms-capabilities tool
LayerMerge now converts all paletted images to RGBA images
fixed import
Skip test if FASTOCTREE quantization is not supported
Skip test if FASTOCTREE quantization is not supported
Minor style fixes
Add test for tms / jpeg
Spelling correction for scales tool.

Fixed typo in --as-res-config help text.
Merge pull request #91 from stevendlander/patch-1

Spelling correction for scales tool.
pass request_format to TileManager

fixes single tiled access from mixed caches to png caches (#52, #94)