vpx_ios/VPXTest: Rename to VPXExample.

- namespace VpxTest -> VpxExample
- dirs/files/projects VPXTest -> VPXExample
- gitignore update: DO NOT ignore VPXExample-Prefix.pch; it's not
  a generated file.
- add VPXExample-Prefix.pch to the repo.
- mark debug/release schemes as shared and add them to the repo.

Change-Id: I7b8fbae87a6ab1368b585c3504b36f0950743858