Expand rake validate task to cover example projects

[blakewatters]

Because of how the project builds and the unit tests link against them, it is possible for header files to be incorrectly configured in the project and all of the unit tests to pass, but projects building against the library to fail outright. We have seen this a few times, including in #299.

We have a rake validate task on tree that is designed to ensure the project builds properly. We should add the building of the example projects to this task and run it before committing to master.