Allow a Condition to be set for an MSBuild Import

A condition specified in the call DotNetProject.AddImportIfMissing(..) will now be saved in the MSBuild project file. This allows imports to be conditional.

One use is for NuGet packages that define a custom MSBuild targets file and allows the targets file to be conditionally imported if the file exists. This pull request also includes a change to the NuGet addin to add Conditions for imports created when installing a NuGet package.

```xml
<Import
  Project="packages\Xamarin.Forms\Xamarin.Forms.targets"
  Condition="Exists('packages\Xamarin.Forms\Xamarin.Forms.targets')"/>
```
Using the Exists check allows Visual Studio to open the project when the package is missing and has not yet been restored. Otherwise Visual Studio will just fail to load the project.