Absolute paths to resources aren't handled correctly

When a resource file (e.g. image, sound) is identified via an absolute path,

1. the file is copied to an appropriate subdirectory of the experiment's server-side "Experiment Cache" directory (good), but
   
2. MWorks still tries to load the file using the original, client-side absolute path (bad).

Item 2 happens because the specialization of mw::ParameterValue::convert that returns a boost::filesystem::path uses mw::expandPath, and mw::expandPath doesn't prepend the working directory to paths that start with /. I think any changes will need to happen in ParameterValue::convert, since expandPath is used both client-side (where absolute paths shouldn't be altered) and server-side (where they should).