Make Oboe::API available even when liboboe.so isn't

Currently, if a production application includes custom profiling code using Oboe::API and a developer goes to execute that code in a Mac development environment, that code will fail with something to the affect of "Oboe::API" is undefined.

This is because the gem goes into a basic no-op mode without liboboe.  Oboe::API should be a part of that no-op mode.