This change is to enable the use of XCode 5 to build the brackets-shell

There were two issues when switching to XCode 5
1) The default architecture was wrong (always 64)
2) The tools/strip_save_dsym script couldn't determine the strip utility, since it got moved to a different location with XCode 5 and the  SYSTEM_DEVELOPER_BIN_DIR environment variable (defined by XCode) points to a different location than in XCode 4

The solution
The XCode project generation is affected in 2 ways:
- set the default architecture to 32Bit
- call a script (scripts/strip_from_xcode.sh) in the post build phase to undef SYSTEM_DEVELOPER_BIN_DIR and call tools/strip_save_dsym afterwards