Add missing nuspec file to Org.Apache.REEF.Bridge.CLR

Add a nuspec file to Org.Apache.REEF.Bridge.CLR since it will be needed when Bridge.CLR becomes the Bridge.

.Net release build is broken in master branch. This is the build error:

"C:\Reef\ReefApache\reef\lang\cs\Org.Apache.REEF.sln" (default target) (1) ->
"C:\Reef\ReefApache\reef\lang\cs\Org.Apache.REEF.Bridge.CLR\Org.Apache.REEF.Bridge.CLR.csproj" (default target) (36) ->
(BuildPackage target) ->
C:\Reef\ReefApache\reef\lang\cs\.nuget\NuGet.targets(148,9): error : Could not find file 'C:\Reef\ReefApache\reef\lang\cs\Org.Apache.REEF.Bridge.CLR\Org.Apache.REEF.Bridge.CLR.
nuspec'. [C:\Reef\ReefApache\reef\lang\cs\Org.Apache.REEF.Bridge.CLR\Org.Apache.REEF.Bridge.CLR.csproj]
C:\Reef\ReefApache\reef\lang\cs\.nuget\NuGet.targets(148,9): error MSB3073: The command ""C:\Reef\ReefApache\reef\lang\cs\.nuget\NuGet.exe" pack "C:\Reef\ReefApache\reef\lang\c
s\\Org.Apache.REEF.Bridge.CLR\Org.Apache.REEF.Bridge.CLR.nuspec" -BasePath C:\Reef\ReefApache\reef\lang\cs
Org.Apache.REEF.Bridge -Properties "Configuration=Release;Platform=x64
;REEF_Version=0.16.0-SNAPSHOT;Version=0.16.0-SNAPSHOT-06" -NonInteractive -OutputDirectory "C:\Reef\ReefApache\reef\lang\cs\\.nuget\packages" " exited with code 1. [C:\Reef\ReefA
pache\reef\lang\cs\Org.Apache.REEF.Bridge.CLR\Org.Apache.REEF.Bridge.CLR.csproj]

If we don't need NuGet for Bridge.CLR, NuGet.targets should not be imported in the csproj.