work in process: mixed mode
added documentation for caching with a mixed mode format
added example for a cache with a mixed mode format
changed comment on mixed mode. CacheConfiguration now checks, if the format is set to mixed and raises an exception if request_format is not set at the same time for the corresponding cache
added function to check image transparency
removed print output
Image format depends on image content, if mixed mode is enabled and the Response will be rendered with the right content-type. Furthermore the format of the cache will be hidden in the Demo service and the Capabilities and exchanged with PNG
set the content-type of the response to tile.format instead of request.format to support mixed mode
changed is_tranparent function, RGBA images will now be checked if they have any alpha values
added WMS, TMS, and WMTS tests for mixed mode
fixed img_has_transparency, now checks if paletted images can be converted to rgba and returns true if any rgba image has transparent pixels
fixed is_transparent, RGBA images are considered to have transparency, if any pixel is not fully opaque
added config for mixed mode tests
fixed assertion error
added tests for function img_has_transparency, which is needed for a mixed mode format
use "new" seed config format in coverage docs
mention support for GeoJSON coverages
fixed rst markup
Merge branch 'master' into mixed_image_mode

Conflicts:
	mapproxy/config/loader.py
add closing </a> in nightly doc header