Remove libextobjc submodule
Add part of libextobjc into Mantle folder as a subtree
Re-add jspahrsummers/xcconfigs as a submodule
Fix up libextobjc and xcconfig references in project file
Rip out unused EXTRuntimeExtensions functionality
Re-namespace libextobjc symbols for Mantle
Remove libextobjc entry from .gitmodules
++xcconfigs to project-headers-first
++xcconfigs
++xcconfigs
Merge pull request #98 from github/hide-libextobjc

Replace libextobjc submodule with a subtree and namespace its symbols