automatically add FCS debug package for next minor.

[matthid]

Suggestion for #762 (comment)

[matthid]

AppVeyor doesn't like me anymore apparently :/

[matthid]

This is ready now. Some notes:

• I don't know why all the targets are so fixed around the bin directory. Ideally we can change the variable in build.fsx and the build still works (currently tests and packaging seem to depend on it)
• We can choose to release the debug version on NuGet (via setting the DEBUG_PACKAGE_PUBLISH environment variable to true)
• We can disable the build of the debug package by setting DEBUG_PACKAGE to false
• The package should be available in the project feed. Someone should probably publish the feed url somewhere (It can be found in the project settings -> NuGet) if we decide to not publish the debug package on NuGet itself.

[dsyme]

Great work, thanks!