allow external image data - refs #2002

Conflicts:
	include/mapnik/image_data.hpp
	tests/cpp_tests/image_io_test.cpp
fix miniz encoder - closes #1560
fix miniz inlining
remove libcurl dependency

Conflicts:
	plugins/input/osm/dataset_deliverer.cpp
	plugins/input/osm/osm.cpp
	plugins/input/osm/osm_datasource.cpp
	plugins/input/osm/test.cpp
webp reader: initialize WebPDecoderConfig

Conflicts:
	src/webp_reader.cpp
improve png comparison tests

Conflicts:
	tests/python_tests/png_encoding_test.py
image readers: ability to check if image has alpha before full read

Conflicts:
	src/webp_reader.cpp
Makefile tweaks for helloworld template plugin

 - Do not force use of clang++ to build the plugin (broken on recent debian
   and derivates: https://bugs.debian.org/643959)
 - Do not force clean on install
 - Use -fPIC to build objects (or g++ complains)
Merge branch 'master' of github.com:mapnik/mapnik