Process or reject more complex Podfile situations

[brunobowden]

It may not be possible for plugin to effectively parse and update the Podfile - particularly when it includes complex ruby scripts. In this situation the plugin should:

1. Identify when it's a non-standard more complex Podfile
2. Process the Podfile if possible
3. If not possible to process, then fail the build with warning about disabling this with Config to only add j2objc_shared method to Podfile and not update targets #561

Example Podfile from this discussion:
https://groups.google.com/forum/#!topic/j2objc-discuss/crJsUkozk3Q

post_install do |installer|
  target = installer.project.targets.find{|t| t.to_s == "Pods-MagicalRecord"}
    target.build_configurations.each do |config|
        s = config.build_settings['GCC_PREPROCESSOR_DEFINITIONS']
        s = [ '$(inherited)' ] if s == nil;
        s.push('MR_ENABLE_ACTIVE_RECORD_LOGGING=0') if config.to_s == "Debug";
        config.build_settings['GCC_PREPROCESSOR_DEFINITIONS'] = s
    end
end