Exclude node_modules (#442)

This fixes building within VS 2017 community.
node_modules can sometimes contain paths that are very long, which can
trigger an msbuild bug

dotnet/msbuild#406

Author: dustinsoftware

src/React.Core/React.Core.csproj

@@ -19,6 +19,12 @@
 	<GenerateAssemblyInfo>false</GenerateAssemblyInfo>
   </PropertyGroup>
 
+  <ItemGroup>
+    <Compile Remove="node_modules\**" />
+    <EmbeddedResource Remove="node_modules\**" />
+    <None Remove="node_modules\**" />
+  </ItemGroup>
+
   <ItemGroup>
     <Compile Include="..\SharedAssemblyInfo.cs" />
     <Compile Include="..\SharedAssemblyVersionInfo.cs" />