Implement Pkg.status(::String)

```
julia> Pkg.status("BinDeps")
 - BinDeps                       0.3.3

julia> Pkg.status()
27 required packages:
 - Calendar                      0.4.2
 - Clang                         0.0.1+             master
 - DICOM                         0.0.0+             master (dirty)
 - DataStructures                0.3.1
 - Devectorize                   0.4.0
...
46 additional packages:
 - Arduino                       0.1.2+             fix2
 - ArrayViews                    0.4.6
 - BinDeps                       0.3.3
 - Cairo                         0.2.16
...
```