Add nuspec file for System.Time library
Add nuspec file for System.Time library

This will allow building a nuget package for System.Time library

VSadov/corefxlab,ahsonkhan/corefxlab,Vedin/corefxlab,nguerrera/corefxlab,hanblee/corefxlab,Vedin/corefxlab,stephentoub/corefxlab,VSadov/corefxlab,Vedin/corefxlab,ericstj/corefxlab,Vedin/corefxlab,Vedin/corefxlab,stephentoub/corefxlab,stephentoub/corefxlab,weshaggard/corefxlab,ericstj/corefxlab,joshfree/corefxlab,ahsonkhan/corefxlab,stephentoub/corefxlab,VSadov/corefxlab,benaadams/corefxlab,whoisj/corefxlab,tarekgh/corefxlab,VSadov/corefxlab,hanblee/corefxlab,stephentoub/corefxlab,livarcocc/corefxlab,KrzysztofCwalina/corefxlab,benaadams/corefxlab,alexperovich/corefxlab,VSadov/corefxlab,VSadov/corefxlab,whoisj/corefxlab,stephentoub/corefxlab,ericstj/corefxlab,ravimeda/corefxlab,dotnet/corefxlab,ericstj/corefxlab,adamsitnik/corefxlab,KrzysztofCwalina/corefxlab,ericstj/corefxlab,adamsitnik/corefxlab,Vedin/corefxlab,joshfree/corefxlab,dotnet/corefxlab,ericstj/corefxlab,tarekgh/corefxlab