Implement completion for package references in csproj files

## Environment data
`dotnet --info` output:
```
.NET Command Line Tools (1.0.0-rc4-004706)

Product Information:
Version: 1.0.0-rc4-004706
Commit SHA-1 hash: e38bc4950c

Runtime Environment:
OS Name: Mac OS X
OS Version: 10.11
OS Platform: Darwin
RID: osx.10.11-x64
Base Path: /usr/local/share/dotnet/sdk/1.0.0-rc4-004706

```
VS Code version: `1.8.1 ee428b0eead68bf0fb99ab5fdc4439be227b6281`
C# Extension version: 1.6.2

## Steps to reproduce
Edit a netcore csproj

## Expected behavior
Intellisense when trying to get nuget package names and versions. This is currently possible in project.json. Editing this file manually without intellisense is a bit painful.

## Actual behavior
No behaviour its just editing a text file