look into making boost::filesystem operations work transparently with Android Asset Manager

[richardeakin]

When porting a project to android, one of the first holes you fall in is that direct access to files doesn't work, instead you need to somehow go through Android Asset Manager. app::loadAsset() and loadFile() have already been made to work, and fs::exists() has also been overloaded on android to query the Android Asset Manager (here).

I think we should add a similar overload for is_directory(), that somehow checks AAssetManager_openDir to see if the directory exists. I'm not sure if there are others that would currently fail but could be made to work. Obviously not all are going to work, or would be very difficult to make work, like directory_iterator for example.

[alias-r-cummins]

This should no longer be necessary if we no longer depend on boost. The question then becomes, 'what is the baseline functionality'?