Change PlatformTarget from x86 to AnyCPU to prevent compiler warning
Remove unused references that are causing compiler warnings
Update System.Collections.Immutable reference from pre-release to current
Removed the System.Collections.Immutable reference from the templates

This meant duplicating the CollectionHelpers methods in the generated
classes (if any fields use collection types) but it will make it easier
to NuGet-up since it won't have a text reference to a particular binary
version
Removed template links from the Tests project that snuck in when I wasn't looking
Change the ImmutableObjectGraph.tt to expect to be part of a NuGet package

This "primary" template will include files from a NuGet installation
while the ".ForTests" template can be used in the Demo and Tests
projects
A single "base" template per-project avoid repeating NuGet references

The Demo project has a "BaseImmutableObjectGraph" template which
references the "primary" ImmutableObjectGraph template and the
System.Collections.Immutable NuGet binary to illustrate how repeating
them can be prevented (and if either NuGet package gets updated, there's
only one place per project to fix by hand)
Replaced hard-coded "bin\debug" with "$(OutDir)" for flexibility/correctness
Demonstrate ignoring of "The field '{0}' is never used" warnings in templates

It's not essential to flag these warnings as ignore-able, but without
doing so there are 48 warnings in building the solution
Tweak to the Builder generator to prevent a warning if the type has no local fields
Change the Release build to target AnyCPU instead of x86 (already done this for Debug)
Enable a NuGet package to be generated from a Release build