Storage Client Library v2.1.0

Changes in v2.1:

• Upgraded Casablanca dependency to 2.6.0
• Added support for Visual Studio 2015, dropped support for Visual Studio 2012
• Separated Nuget packages to wastorage.v120 and wastorage.v140. For backwards compatibility, wastorage became an umbrella package which has both packages as dependencies.
• Retired CoApp autopkg. Nuget.exe and nuspec files are used to generate Nuget packages.
• Fixed an issue that MSBuild must run twice to first download Nuget packages then compile.