Add ofGetVersionMajor(), ofGetVersionMinor() to ofUtils

They return `OF_VERSION` and `OF_VERSION_MINOR` respectively. I wrote them because [Zajal](https://github.com/nasser/zajal) uses openFrameworks as [a shared library](https://github.com/nasser/zajal/tree/amsterdam/lib/core/lib), and can only see functions, not `#define`s. 

Although you can use the `#define`s from normal openFrameworks C++ projects, this patch brings internal version information under the more standard syntax.