Fixed issue where the generated Archive was empty.

Locally override the value of INSTALL_PATH that we picked up from the Omni
xcconfig file which was causing the generated Archive to be empty.
Fix for issue #11: Can't archive for for App Store Upload

Rework how the GraphSketcherModel headers are copied so they are in an
accessible location when generating the PCH during a Release+Archive build.

Locally override the value of INSTALL_PATH that we picked up from the Omni
xcconfig file which was causing the generated Archive to be empty.