Inline build script property $project ("Fixie"), to improve readability and to avoid conflicting with variables in an upcoming commit in which the word 'project' will be use to refer to csproj files within the solution.
Instead of sharing a generated CommonAssemblyInfo.cs file across all projects, each project's own AssemblyInfo.cs is written in full by the build script.  This way, a developer adding a new project does not have to remember to take part in AssemblyInfo generation.
Generate AssemblyInfo.cs files in UTF8 so that the copyright character can be used.
For readability, build script uses multiline string when writing AssemblyInfo.cs files.
Regenerate an AssemblyInfo.cs file only when the content is actually changing, to minimize noise in the build script output.
Build script updates LICENSE.txt upon the first build of each new year, so the copyright date span is always accurate.
Drop some irrelevant/redundant details from AssemblyInfo.cs files.
Build script sanity checks that all projects intended for publication take part in the special build output path ..\..\build.  Otherwise, a new project added to the solution will likely silently fail to take part in builds.  Self-test projects Fixie.Tests and Fixie.Samples are excluded from the check and may have 'normal' build output paths, since they do not need to be deployed.
Merge pull request #62 from plioi/BuildScriptEnhancements

Build script enhancements.