Added simple AzureTableBrain nuspec based on RedisBrain's nuspec

This should allow you to "nuget pack" and publish it to nuget.org like you did with http://www.nuget.org/packages/MMBot.RedisBrain/

Once it's on nuget.org, it'll make it easier to get started with mmbot with an Azure Table brain via:

    nuget install MMBot.AzureTableBrain -o packages

I tested the "nuget pack" locally and it works great.

I didn't go ahead an publish it to nuget.org since @PeteGoo publishes most MMBot related ones, so he or the original author, Sam Wood ( @Spksh ), deserves the credit. Feel free to publish it if you decide to accept this PR.

Thanks for all your work on mmbot. We really enjoy using it!