Update nuspec files

- Description has been moved to each nuspec file instead of having them in build.fsx
- Switched to description instead of summary. If summary is missing
description is used instead.
See http://docs.nuget.org/docs/reference/nuspec-reference
- Added additional tags where relevant
- Added BETA to every package's title. To remove set
nugetTitleSuffix = "" in build.fsx