Added a find-refs function (Issue #333)

This PR adds a find-refs function that allows to specify a list of Nuget package names and get all the paket-references back that contain them.

you simply run `paket find-refs --packages PACKAGENAME1 PACKAGENAME1 ...`

and get a result like this:

```
DotNetZip
.src/Paket.Core/paket.references

FSharp.Core.Microsoft.Signed
.src/Paket.Core/paket.references
.src/Paket/paket.references
```

Docs are included.