Add the necessary directories to the include path within xcode

This is not necessary when using precompiled headers.
Remove unnecessary precompiled headers

This reduces the huge amount of near-useless small extra files
scattered around for the sake of the platform demo projects.

This requires explicitly listing all the ncessary include paths.
Merge pull request #349 from mstorsjo/cleanup-ios-project

Remove unnecessary precompiled headers from the xcode projects