Adding property_tree

[SVN r37680]

.gitmodules

@@ -250,6 +250,10 @@
 	path = libs/algorithm
 	url = ../algorithm.git
 	fetchRecurseSubmodules = on-demand
+[submodule "property_tree"]
+	path = libs/property_tree
+	url = ../property_tree.git
+	fetchRecurseSubmodules = on-demand
 [submodule "xpressive"]
 	path = libs/xpressive
 	url = ../xpressive.git

libs/property_tree

@@ -0,0 +1 @@
+Subproject commit f4105f823f6a40f0b8967d3c72ed51f81ac6d9e6