mapnik::value

to_expression_string - add missing specialization
for mapnik::value_integer and remove usage of std::stringstream
remove uneeded python imports
Add test for image.fromstring - refs #1805
commment failing tiff test - refs #1831
Add tests for frombuffer as well - refs #1805
reduce libary size by passing -fvisibility-inlines-hidden - closes #1832 - refs #1826
no need to use MAPNIK_DECL with typedefs
make public the singleton template to ensure it can be shared across shared library boundaries if the library is built with -fvisibility=hidden - refs #1826
make mapped_memory_cache a class not struct for consistency with other singletons
fix build on windows by ensuring singleton base is exported (avoids inconsistent dll linkage error)
support visibility attribute with gcc/clang - refs #1826
-fvisibility=hidden works with plugins with no code changes - refs #1826
remove kismet plugin - closes #1833
upgrade clipper to 5.1.5 - refs #1749
no need to cache config error specifically
support -fvisibility=hidden in core library - refs #1826
+ fix writing to std::stringstream

* if seeking pass the end  - grow buffer (stringstream!)
* seekp beyond current buffer size doesn't set failbit (clang/libcxx)
default to clang++ on OS X and start reporting CXX compiler used in mapnik-config - closes #1839
add --all-flags to mapnik-config - closes #1839
add mapnik version number reporting to mapnik-config