Convert .Rproj file extension to associated UTI on 'Open Project'

This fixes a bug wherein spotlight search failed within 'Open Project'... dialogs. If the file extension is `.Rproj`, we pass the associated Uniform Type Identifier, rather than the extension itself, so that search functions correctly.

NOTE: I took the UTI from a sample `.Rproj` file with e.g. `mdls devtools.Rproj`, which gave back:

```
<...>
kMDItemContentType             = "dyn.ah62d4rv4ge81e6dwr7za"
kMDItemContentTypeTree         = (
    "dyn.ah62d4rv4ge81e6dwr7za",
    "public.data",
    "public.item"
)
<...>
```

and so that is how I obtained the ID; however, I think there must be some better method of programmatically getting the UTI, but I haven't been able to figure it out. But we should double-check that it's consistent across different OS X versions.

EDIT: Using a [custom base32 decoder](http://play.golang.org/p/sW5oTXs4Bl), we can see that the string encodes:

    ?0=6:1=rproj

which is a compact way of storing from identifier information, alongside the extension (hence why it works)