Change Get-Projects to return the default project if no name is given.

This is the original (and probably expected) behavior.
Ensure that assembly reference names are found in the hashtable.

Convert names to uppercase to ensure that a case-insensitive search is
used when looking up managed DLL paths.
Silence the output of setting the HintPath metadata.
Add the Add-Unity3DReference command.
Add the Remove-Unity3DReference command.
Add missing nuspec entries and flesh out the description.
Add a copyright element and empty references element to the nuspec.
Add a reference to the UnityEngine assembly on installation.
Check in PluginTest with latest installed Unity3D.DLLs package.
Fill out README.md.
Add initial documentation generated from Markdown docs.
Update the summary, project, and license URLs in the nuspec.
Fix a bug where fully-qualified assembly names were skipped during
update.
Set the project ReferencePath value directly so that it is immediately
reflected in the IDE and project system.
Move the MSBuild ReferencePath property code into functions that can
either set it directly or go through the MSBuild project.
Ensure that the reference path is up-to-date even if a reference already
exists.
Skip updating project references on install if the Unity3DSkipAutoUpdate
property isn't true.
Complete the copyright banners to fix #2.
Add a link to the GitHub project and make the email link explicit.
Add a changelog.
Add the project pages source to the solution.