Update gitignore to be effective.
Factorize build settings in external xcconfig files.
Doing this greatly reduces the number of duplicated settings, as 'SDK' specific settings can be specified only once instead of once per Build Configuration.

This also fix a couple of issues with 10.9 SDK and python (which was already fixed for some targets but not all).
Project cleanup
Add debugger packages support.
Useful args from debugserver

Summary:

Reviewers:

CC: