add support for halo-rasterizer for shields - closes #2226

Conflicts:
	src/load_map.cpp
	tests/visual_tests/images/shield-on-line-spacing-eq-width-600-400-1.0-agg-reference.png
	tests/visual_tests/images/shield-on-line-spacing-eq-width-600-400-2.0-agg-reference.png
optimize string2bool
update expecte test images accidentally overwritten from 2.3.x merge
fix return value for string2bool
c++11 optimizations for faster map loading
cache result of parse_expression to speed up map loading - refs #1167
array alloc bench: add static_vector + fix valarray usage
inherit PYTHONPATH in localize.sh
Add serialization for group symbolizer components.
Merge pull request #2230 from MapQuest/jh-fix-group-symbolizer-text

Group symbolizer serialization
rapidxml: validate closing tags like libxml2 parser (no overhead measurable seen)
more realistic expression benchmark string - refs #1167
static grammars
 - construct on first use
 - allows for faster map loading and unloading of xml loading tree
 - modified expression and transform grammars to not take args/not crash
 - simplifies interfaces, allows fast parsing from python without passing grammar instance
all static grammar usage const + xml_tree cleanups
cleanup + rename svg_transform parser
add TODO comments for grammars that are not stateless - refs #2231
benchmark: png encoding with z=1 to ensure we're testing hextree not zlib
benchmark: break out string to int/bool/double conversions
make clean: also clear out shared libs
fix variable shadowing in placements_list.hpp