[Podspec] Cleanup and doc

- Remove note about HEADER_SEARCH_PATHS from description
- Clean up excludes
- Add comments and previous attempts to fix build issues inside the xcconfig section
- Add explicit dependency on zlib
Rename entry struct to git_entry

Side-steps struct name conflict with global 'entry' in search.h below:

/Applications/Xcode6.1.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS8.1.sdk/usr/include/search.h (line 16)