added .gitignore for Xcode project files
Fixed graph generator runtime errors
Fixed another bug
Added Google sparsehash and debugged backbone
Fixed typo in edge bundling
Fixed memory leak
Additional resources
Made .gitignore for Xcode project, modified makefile

Manually specified include and linking libraries for CPLEX in makefile
fixed typo
pulling changes from master into sign_enumeration
cleaning up sign enumeration function
- working on integration of rev. cuthill-mckee
- making more elements of tree pointers