Change NuGet package native dlls deployment mode

Latest NuGet version (2.5) introduces a feature that should help the deployment of native dlls: **[Automatic import of msbuild targets and propos files](http://docs.nuget.org/docs/release-notes/nuget-2.5#Automatic_import_of_msbuild_targets_and_props_files)**.

This could help removing the `nuget.package/Tools/*.ps1` scripts by adding a `.targets` file in the `/build/<framework>` package folder, which can contain <itemgroup> tags that have items that are set to copy to output.

A sample is provided below:

```xml
<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" InitialTargets="zlib_redist_init">


<ItemGroup >
    <None Include="$(MSBuildThisFileDirectory)..\..\build\native\bin\x64\v110\dynamic\cdecl\Release\zlib1.dll" >
      <DeploymentContent>true</DeploymentContent>
    </None>
</ItemGroup>

</Project>
```
Some additional low-level information on the topic: **[How to Build a Package](http://coapp.org/tutorials/building-a-package.html)**

**Note:** We may also add some **[minimum NuGet Client version requirement](http://docs.nuget.org/docs/release-notes/nuget-2.5#Add_a_%27Minimum_NuGet_Version%27_property_to_packages)**, but it is unclear how a NuGet client < 2.5 will react to this directive and to the additional `.targets` file