restore the references just for the project

Thanks to #267 and the below changes, our build time decreased by only downloading required nuget packages. Our `build.cmd` now looks like:
```
.paket\paket.bootstrapper.exe
.paket\paket.exe restore --references-files paket.references
packages\FAKE\tools\FAKE.exe build.fsx %*
```
we have a second version of this file  `myproject.csproj.paket.references` which is a special version if you have more than one project in the folder - can you also check for this?