Add support for VS projects and solutions

[CharliePoole]

This is a feature already included in NUnit 2.6 and at least partially implemented in the engine already.

• Running a VS Project should work the same as running the output assembly from the project. If a project type is not one we support, an error message should result.
• Running a Solution should be equivalent to entering the names of all the output assemblies on the command line. Any project types we don't support can be skipped without notice. Filtering for test assemblies supported by NUnit 3 is an option if it can be done easily using the engine.
• This issue in the old nunit-console project can be dealt with at the same time as this one by ignoring any projects that are disabled in the configuration: Make everything build after big split nunit-console#1

[rprouse]

When we do this, make sure that we support F# projects. An issue was entered reporting that they do not work in 2.6.3.

nunit/nunit-console#43

[CharliePoole]

Another note: We should decide what to do about project types we don't recognize. There are pitfalls either way we go.