c++ style
memory_datasource:  support datasource interface
share datasources between layers for now
preserve old MemoryDatasource interface in Python
mark is_open as const
create shape_io object per featureset to allow datasource sharing
format
remove unused anonymous namespace
c++ formatting
feature_impl : make context() method const
pedantic : use const quialifier where appropriate
more const correctness + cleaups
support for GDAL >= 2.0 in ogr plugin - refs #2259
improvements to GDAL 2.x handling - refs #2259 amends cbdd112223b42576
fix spelling in comment
update travis script
initial refactor pass (fixes #2235)
remove dead code
re-enable clang since #2235 should be fixed soon
add benchmark of utf 32->8 encoding speed