added strong name to assembly

[yesmarket]

Hello Mehdi,

I read your comprehensive blog about DbContextScope and I was impressed! I started using the library in a couple of my projects. I was also creating a "query-by-specification" framework and wanted to use this as part of an Entity Framework integration library. I was planning to sign my assembly and as such needed DbContextScope to also be signed. Please accept the pull request.

Also thank you for creating this useful library :)

Ryan.