Update version of dotnet.exe used to build + provide wrapper script to create clurun + command packages

[johanste]

The version of the dotnet toolset used to test the scripts can be found at: https://azureclu.blob.core.windows.net/tools/dotnet-win-x64.latest.zip

This required minor tweaks to BuildProfile as the behavior for which file end up in the published output has changed (the content directory and nuspec files were no longer included in the published output)

Add a BuildDrop.ps1 file that will do a build and local publish of clurun for osx, linux/ubuntu and win7 along with any command packages in the repo.

Please note that the build scripts will not currently copy the msclu.cfg file to the clurun directory - that, along with any updates needed (i.e. package repo location) has to be done manually.

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[yugangw-msft]

@johanste please update .gitignore to skip the drop folder